One document matched: draft-rescorla-sec-cons-01.txt
Differences from draft-rescorla-sec-cons-00.txt
E. Rescorla
RTFM, Inc.
B. Korver
INTERNET-DRAFT Network Alchemy
<draft-rescorla-sec-cons-01.txt> (June 2000 (Expires December 2000)
Guidelines for Writing RFC Text on Security Considerations
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference mate-
rial or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Introduction
All RFCs are required by [RFC1543] to contain a Security Considera-
tions section. The purpose of this is both to encourage document
authors to consider security in their designs and to inform the
reader of relevant security issues. This memo is intended to provide
guidance to RFC authors in service of both ends.
This document is structured in three parts. The first is a combina-
tion security tutorial and definition of common terms; the second is
a series of guidelines for writing Security Considerations; the third
is a series of examples.
2. The Goals of Security
Most people speak of security as if it were a single monolithic prop-
erty of a protocol or system, but upon reflection that's very clearly
not true. Rather, security is a series of related but somewhat inde-
pendent properties. Not all of these properties are required for
every application.
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We can loosely divide security goals into those related to protecting
communications (COMMUNICATIONS SECURITY) and those relating to pro-
tecting systems (SYSTEMS SECURITY). Since communications are carried
out by systems and access to systems is through communications chan-
nels, these goals obviously interlock, but they can also be indepen-
dently provided.
2.1. Communications Security
Different authors partition the goals of communications security dif-
ferently. The partitioning we've found most useful is to divide them
into three major categories: CONFIDENTIALITY, DATA INTEGRITY and END-
POINT AUTHENTICATION.
2.1.1. Confidentiality
When most people think of security, they think of CONFIDENTIALITY.
Confidentiality means that your data is kept secret from unintended
listeners. Usually, these listeners are simply eavesdroppers. When
the government taps your phone, that poses a risk to your confiden-
tiality.
Obviously, if you have secrets, you're concerned that no-one else
knows them and so at minimum you want confidentiality. When you see
spies in the movies go into the bathroom and turn on all the water to
foil bugging, the property they're looking for is confidentiality.
2.1.2. Data Integrity
The second primary goal is DATA INTEGRITY. The basic idea here is
that we want to be sure that the data we receive is the one that the
sender sent. In paper-based systems, some data integrity comes auto-
matically. When you receive a letter written in pen you can be fairly
certain that no words have been removed by an attacker because pen
marks are difficult to remove from paper. However, an attacker could
have easily added some marks to the paper and completely changed the
meaning of the message. Similarly, it's easy to shorten the page to
truncate the message.
On the other hand, in the electronic world, since all bits look
alike, it's trivial to tamper with messages in transit. You simply
remove the message from the wire, copy out the parts you like, add
whatever data you want, and generate a new message of your choosing,
and the recipient is no wiser. This is the moral equivalent of the
attacker taking a letter you wrote, buying some new paper and recopy-
ing the message, changing it as he does it. It's just a lot easier to
do electronically since all bits look alike.
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2.1.3. Endpoint authentication
The third property we're concerned with is ENDPOINT AUTHENTICATION.
What we mean by this is that we know that one of the endpoints in the
communication is the one we intended. Without endpoint authentica-
tion, it's very difficult to provide either confidentiality or data
integrity. For instance, if we receive a message from Alice, the
property of data integrity doesn't do us much good unless we know
that it was in fact sent by Alice and not the attacker. Similarly, if
we want to send a confidential message to Bob, it's not of much value
to us if we're actually sending a confidential message to the
attacker.
Note that endpoint authentication can be provided asymmetrically.
When you call someone on the phone, you can be fairly certain that
you have the right person -- or at least that you got a person who's
actually at the phone number you called. On the other hand, if they
don't have caller ID, then the receiver of a phone call has no idea
who's calling them. Calling someone on the phone is an example of
recipient authentication, since you know who the recipient of the
call is, but they don't know anything about the sender.
On the other hand, cash is an example of sender authentication. A
dollar bill is like a message signed by the government. The govern-
ment has no idea who's got any given dollar bill but you can be
fairly certain that any bill was actually printed by the US Mint
because currency is difficult to forge.
2.2. Systems Security
In general, systems security is concerned with protecting one's
machines and data. The intent is that machines should be used only by
authorized users and for the purposes that the owners intend. Fur-
thermore, they should be available for those purposes. Attackers
should not be able to deprive legitimate users of resources.
2.2.1. Unauthorized Usage
Most systems are not intended to be completely accessible to the pub-
lic. Rather, they are intended to be used only by certain authorized
individuals. Although many Internet services are available to all
Internet users, even those servers generally offer a larger subset of
services to specific users. For instance, Web Servers often will
serve data to any user, but restrict the ability to modify pages to
specific users. Such modifications by the general public would be
UNAUTHORIZED USAGE.
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2.2.2. Inappropriate Usage
Being an authorized user does not mean that you have free run of the
system. As we said above, some activities are restricted to autho-
rized users, some to specific users, and some activities are gener-
ally forbidden to all but administrators. Moreover, even activities
which are in general permitted might be forbidden in some cases. For
instance, users may be permitted to send email but forbidden from
sending files above a certain size, or files which contain viruses.
These are examples of INAPPROPRIATE USAGE.
2.2.3. Denial of Service
Recall that our third goal was that the system should be available to
legitimate users. A broad variety of attacks are possible which
threaten such usage. Such attacks are collectively referred to as
DENIAL OF SERVICE attacks. Denial of service attacks are often very
easy to mount and difficult to stop. Many such attacks are designed
to consume machine resources, making it difficult or impossible to
serve legitimate users. Other attacks cause the target machine to
crash, completely denying service to users.
3. The Internet Threat Model
A THREAT MODEL describes the capabilities that an attacker is assumed
to be able to deploy against a resource. It should contain such
information as the resources available to an attacker in terms of
information, computing capability, and control of the system. The
purpose of a threat model is twofold. First, we wish to identify the
threats we are concerned with. Second, we wish to rule some threats
explicitly out of scope. Nearly every security system is vulnerable
to a sufficiently dedicated and resourceful attacker.
The Internet environment has a fairly well understood threat model.
In general, we assumed that the end-systems engaging in a protocol
exchange have not themselves been compromised. Protecting against an
attack when one of the end-systems has been compromised is extraordi-
narily difficult. It is, however, possible to design protocols which
minimize the extent of the damage done under these circumstances.
By contrast, we assume that the attacker has nearly complete control
of the communications channel over which the end-systems communicate.
This means that the attacker can read any PDU (Protocol Data Unit) on
the network and undetectably remove, change, or inject forged packets
onto the wire. This includes being able to generate packets that
appear to be from a trusted machine. Thus, even if the end-system
with which you wish to communicate is itself secure, the Internet
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environment provides no assurance that packets which claim to be from
that system in fact are.
It's important to realize that the meaning of a PDU is different at
different levels. At the IP level, a PDU means an IP packet. At the
TCP level, it means a TCP segment. At the application layer, it means
some kind of application PDU. For instance, at the level of email, it
might either mean an RFC-822 message or a single SMTP command. At the
HTTP level, it might mean a request or response.
3.1. Limited Threat Models
As we've said, a resourceful and dedicated attacker can control the
entire communications channel. However, a large number of attacks can
be mounted by an attacker with fewer resources. A number of cur-
rently known attacks can be mounted by an attacker with limited con-
trol of the network. For instance, password sniffing attacks can be
mounted by an attacker who can only read arbitrary packets. This is
generally referred to as a PASSIVE ATTACK.
By contrast, Morris's sequence number guessing attack [REF] can be
mounted by an attacker who can write but not read arbitrary packets.
Any attack which requires the attacker to write to the network is
known as an ACTIVE ATTACK.
Thus, a useful way of organizing attacks is to divide them based on
the capabilities required to mount the attack. The rest of this sec-
tion describes these categories and provides some examples of each
category.
3.2. Passive Attacks
In a passive attack, the attacker reads packets off the network but
does not write them. The simplest way to mount such an attack is to
simply be on the same LAN as the victim. On most common LAN configu-
rations, including Ethernet, 802.3, and FDDI, any machine on the wire
can read all traffic destined for any other machine on the same LAN.
Note that switching hubs make this sort of sniffing substantially
more difficult, since traffic destined for a machine only goes to the
network segment which that machine is on.
Similarly, an attacker who has control of a host in the communica-
tions path between two victim machines is able to mount a passive
attack on their communications. It is also possible to compromise
the routing infrastructure to specifically arrange that traffic
passes through a compromised machine. This might involve an active
attack on the routing infrastructure to facilitate a passive attack
on a victim machine.
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Wireless communications channels deserve special consideration.
Since the data is simply broadcast on well-known radio frequencies,
an attacker simply needs to be able to receive those transmissions.
Such channels are especially vulnerable to passive attacks.
In general, the goal of a passive attack is to obtain information
which the sender and receiver would rather remain private. Examples
of such information include credentials useful in the electronic
world such as passwords or credentials useful in the outside world,
such as confidential business information.
3.2.1. Privacy Violations
The classic example of passive attack is sniffing some inherently
private data off of the wire. For instance, despite the wide avail-
ability of SSL, many credit card transactions still traverse the
Internet in the clear. An attacker could sniff such a message and
recover the credit card number, which can then be used to make fraud-
ulent transactions. Moreover, confidential business information is
routinely transmitted over the network in the clear in email.
3.2.2. Password Sniffing
Another example of a passive attack is PASSWORD SNIFFING. Password
sniffing is directed towards obtaining unauthorized use of resources.
Many protocols, including TELNET [REF], POP [REF], and NNTP [REF],
use a shared password to authenticate the client to the server. Fre-
quently, this password is transmitted from the client to the server
in the clear over the communications channel. An attacker who can
read this traffic can therefore capture the password and REPLAY it.
That is to say that he can initiate a connection to the server and
pose as the client and login using the captured password.
Note that although the login phase of the attack is active, the
actual password capture phase is passive. Moreover, unless the server
checks the originating address of connections, the login phase does
not require any special control of the network.
3.2.3. Offline Cryptographic Attacks
Many cryptographic protocols are subject to OFFLINE ATTACKS. In such
a protocol, the attacker recovers data which has been processed using
the victim's secret key and then mounts a cryptanalytic attack on
that key. Passwords make a particularly vulnerable target because
they are typically low entropy. A number of popular password-based
challenge response protocols are vulnerable to DICTIONARY SEARCH. The
attacker captures a challenge-response pair and then proceeds to try
entries from a list of common words (such as a dictionary file) until
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he finds a password that produces the right response.
A similar such attack can be mounted on a local network when NIS is
used. The Unix password is crypted using a one-way function, but
tools exist to break such crypted passwords [REF: Crack]. When NIS
is used, the crypted password is transmitted over the local network
and an attacker can thus sniff the password and attack it.
Historically, it has also been possible to exploit small operating
system security holes to recover the password file using an active
attack. These holes can then be bootstrapped into an actual account
by using the aforementioned offline password recovery techniques.
Thus we combine a low-level active attack with an offline passive
attack.
3.3. Active Attacks
When an attack involves writing data to the network, we refer to this
as an ACTIVE ATTACK. When IP is used without IPSEC, there is no
authentication for the sender address. As a consequence, it's
straightforward for an attacker to create a packet with a source
address of his choosing. We'll refer to this as a SPOOFING ATTACK.
Under certain circumstances, such a packet may be screened out by the
network. For instance, many packet filtering firewalls screen out all
packets with source addresses on the INTERNAL network that arrive on
the EXTERNAL interface. Note, however, that this provides no protec-
tion against an attacker who is inside the firewall. In general,
designers should assume that attackers can forge packets.
However, the ability to forge packets does not go hand in hand with
the ability to receive arbitrary packets. In fact, there are active
attacks that involve being able to send forged packets but not
receive the responses. We'll refer to these as BLIND ATTACKS.
Note that not all active attacks require forging addresses. For
instance, the TCP SYN denial of service attack [REF] can be mounted
successfully without disguising the sender's address. However, it is
common practice to disguise one's address in order to conceal one's
identity if an attack is discovered.
Each protocol is susceptible to specific active attacks, but experi-
ence shows that a number of common patterns of attack can be adapted
to any given protocol. The next sections describe a number of these
patterns and give specific examples of them as applied to known pro-
tocols.
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3.3.1. Replay Attacks
In a REPLAY ATTACK, the attacker records a sequence of messages off
of the wire and plays them back to the party which originally
received them. Note that the attacker does not need to be able to
understand the messages. He merely needs to capture and retransmit
them.
For example, consider the case where an S/MIME message is being used
to request some service, such as a credit card purchase or a stock
trade. An attacker might wish to have the service executed twice, if
only to inconvenience the victim. He could capture the message and
replay it, even though he can't read it, causing the transaction to
be executed twice.
3.3.2. Message Insertion
In a MESSAGE INSERTION attack, the attacker forges a message with
some chosen set of properties and injects it into the network. Often
this message will have a forged source address in order to disguise
the identity of the attacker.
For example, a denial-of-service attack can be mounted by inserting a
series of spurious TCP SYN packets [REF] directed towards the target
host. The target host responds with its own SYN and allocates kernel
data structures for the new connection. The attacker never completes
the 3-way handshake, so the allocated connection endpoints just sit
there taking up kernel memory. Typical TCP stack implementations only
allow some limited number of connections in this "half-open" state
and when this limit is reached, no more connections can be initiated,
even from legitimate hosts. Note that this attack is a blind attack,
since the attacker does not need to process the victim's SYNs.
3.3.3. Message Deletion
In a MESSAGE DELETION attack, the attacker removes a message from the
wire. Morris's sequence number guessing attack [REF] often requires
a message deletion attack to be performed successfully. In this blind
attack, the host whose address is being forged will receive a spuri-
ous TCP SYN packet from the host being attacked. Receipt of this SYN
packet generates a RST, which would tear the illegitimate connection
down. In order to prevent this host from sending a RST so that the
attack can be carried out successfully, Morris describes flooding
this host to create queue overflows such that the SYN packet is lost
and thus never responded to.
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3.3.4. Message Modification
In a MESSAGE MODIFICATION attack, the attacker removes a message from
the wire, modifies it, and reinjects it into the network. This sort
of attack is particularly useful if the attacker wants to send some
of the data in the message but also wants to change some of it.
Consider the case where the attacker wants to attack an order for
goods placed over the Internet. He doesn't have the victim's credit
card number so he waits for the victim to place the order and then
replaces the delivery address (and possibly the goods description)
with his own. Note that this particular attack is known as a CUT-AND-
PASTE attack since the attacker cuts the credit card number out of
the original message and pastes it into the new message.
3.3.5. Man-In-The-Middle
A MAN-IN-THE-MIDDLE attack combines the above techniques in a special
form: The attacker subverts the communication stream in order to pose
as the sender to receiver and the receiver to the sender:
What Alice and Bob think:
Alice <----------------------------------------------> Bob
What's happening:
Alice <----------------> Attacker <----------------> Bob
This differs fundamentally from the above forms of attack because it
attacks the identity of the communicating parties, rather than the
data stream itself. Consequently, many techniques which provide
integrity of the communications stream are insufficient to protect
against man-in-the-middle attacks.
Man-in-the-middle attacks are possible whenever a protocol lacks
MUTUAL ENDPOINT AUTHENTICATION. For instance, if an attacker can
hijack the client TCP connection during the TCP handshake (perhaps by
responding to the client's SYN before the server does), then the
attacker can open another connection to the server and begin a man-
in-the-middle attack.
4. Common Issues
Although each system's security requirements are unique, certain com-
mon requirements appear in a number of protocols. Often, naive proto-
col designers are faced with these requirements, they choose an obvi-
ous but insecure solution even though better solutions are available.
This section describes a number of issues seen in many protocols and
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the common pieces of security technology that may be useful in
addressing them.
4.1. User Authentication
Essentially every system which wants to control access to its
resources needs some way to authenticate users. A nearly uncountable
number of such mechanisms have been designed for this purpose. The
next several sections describe some of these techniques. [HA94] cov-
ers this topic in detail.
4.1.1. Username/Password
The most common access control mechanism is simple USERNAME/PASSWORD
The user provides a username and a reusable password to the host
which he wishes to use. This system is vulnerable to a simple passive
attack where the attacker sniffs the password off the wire and then
initiates a new session, presenting the password. This threat can be
mitigated by hosting the protocol across an encrypted connection such
as TLS or IPSEC. Unprotected (plaintext) username/password systems
are not acceptable in IETF standards.
4.1.2. Challenge Response and One Time Passwords
Systems which desire greater security than USERNAME/PASSWORD often
employ either a ONE TIME PASSWORD [OTP] scheme or a CHALLENGE-
RESPONSE. In a one time password scheme, the user is provided with a
list of passwords, which must be used in sequence, one time each.
(Often these passwords are generated from some secret key so the user
can simply compute the next password in the sequence.) SecureID and
DES Gold are variants of this scheme. In a challenge-response scheme,
the host and the user share some secret (which often is represented
as a password). In order to authenticate the user, the host presents
the user with a (randomly generated) challenge. The user computes
some function based on the challenge and the secret and provides that
to the host, which verifies it. Often this computation is performed
in a handheld device, such as a DES Gold card.
Both types of scheme provide protection against replay attack, but
often still vulnerable to an OFFLINE KEYSEARCH ATTACK (a form of pas-
sive attack): As previously mentioned, often the one-time password or
response is computed from a shared secret. If the attacker knows the
function being used, he can simply try all possible shared secrets
until he finds one that produces the right output. This is made eas-
ier if the shared secret is a password, in which case he can mount a
DICTIONARY ATTACK--meaning that he tries a list of common words (or
strings) rather than just random strings.
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These systems are also often vulnerable to an active attack. Unless
communications security is provided for the entire session, the
attacker can simply wait until authentication has been performed and
hijack the connection.
4.1.3. Certificates
A simple approach is to have all users have certificates [PKIX] which
they then use to authenticate in some protocol-specific way, as in
[TLS] or [S/MIME]. The primary obstacle to this approach in client-
server type systems is that it requires clients to have certificates,
which can be a deployment problem.
4.1.4. Some Uncommon Systems
There are ways to do a better job than the schemes mentioned above,
but they typically don't add much security unless communications
security (at least message integrity) will be employed to secure the
connection, because otherwise the attacker can merely hijack the con-
nection after authentication has been performed. A number of proto-
cols ([EKE], [SPEKE], [SRP2]) allow one to securely bootstrap a
user's password into a shared key which can be used as input to a
cryptographic protocol. Similarly, the user can authenticate using
public key certificates. (e.g. S-HTTP client authentication). Typi-
cally these methods are used as part of a more complete security pro-
tocol.
4.1.5. Host Authentication
Host authentication presents a special problem. Quite commonly, the
addresses of services are presented using a DNS hostname, for
instance as a URL [REF]. When requesting such a service, one has
ensure that the entity that one is talking to not only has a certifi-
cate but that that certificate corresponds to the expected identity
of the server. The important thing to have is a secure binding
between the certificate and the expected hostname.
For instance, it is usually not acceptable for the certificate to
contain an identity in the form of an IP address if the request was
for a given hostname. This does not provide end-to-end security
because the hostname-IP mapping is not secure unless secure DNS [REF]
is being used. This is a particular problem when the hostname is pre-
sented at the application layer but the authentication is performed
at some lower layer.
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4.2. Authorization vs. Authentication
AUTHORIZATION is the process by which one determines whether an
authenticated party has permission to access a particular resource or
service. Although tightly bound, it is important to realize that
authentication and authorization are two separate mechanisms. Per-
haps because of this tight coupling, authentication is sometimes mis-
takenly thought to imply authorization. Authentication simply iden-
tifies a party, authorization defines whether they can perform a cer-
tain action.
Authorization necessarily relies on authentication, but authentica-
tion alone does not imply authorization. Rather, before granting
permission to perform an action, the authorization mechanism must be
consulted to determine whether that action is permitted.
4.2.1. Access Control Lists
One common form of authorization mechanism is an access control list
(ACL) that lists users that are permitted access to a resource. Since
assigning individual authorization permissions to each resource is
tedious, often resources are hierarchically arranged such that the
parent resource's ACL is inherited by child resources. This allows
administrators to set top level policies and override them when nec-
essary.
4.2.2. Certificate Based Systems
While the distinction between authentication and authorization is
intuitive when using simple authentication mechanisms such as user-
name and password (i.e., everyone understands the difference between
the administrator account and a user account), with more complex
authentication mechanisms the distinction is sometimes lost.
With certificates, for instance, presenting a valid signature does
not imply authorization. The signature must be backed by a certifi-
cate chain that contains a trusted root, and that root must be
trusted in the given context. For instance, users who possess cer-
tificates issued by the Acme MIS CA may have different web access
privileges than users who possess certificates issued by the Acme
Accounting CA, even though both of these CAs are "trusted" by the
Acme web server.
Mechanisms for enforcing these more complicated properties have not
yet been completely explored. One approach is simply to attack poli-
cies to ACLs describing what sorts of certificates are trusted.
Another approach is to carry that information with the certificate,
either as a certificate extension/attribute [PKIX, SPKI] or as a
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separate "Attribute Certificate".
4.3. Providing Traffic Security
Securely designed protocols should provide some mechanism for secur-
ing (meaning integrity protecting, authenticating, and possibly
encrypting) all sensitive traffic. One approach is to secure the pro-
tocol itself, as in Secure DNS [REF], S/MIME [REF] or S-HTTP [REF].
Although this provides security which is most fitted to the protocol,
it also requires considerable effort to get right.
Many protocols can be adequately secured using one of the available
channel security systems. We'll discuss the two most common, IPSEC
[REF] and SSL/TLS [REF].
4.3.1. IPSEC
The IPsec protocols (specifically, AH and ESP) can provide transmis-
sion security for all traffic between two hosts. The IPsec protocols
support varying granularities of user identification, including for
example "IP Subnet", "IP Address", "Fully Qualified Domain Name", and
indivudual user ("Mailbox name"). However, a given IPsec implementa-
tion might not support all identity types. For example, an encrypt-
ing security gateway usually does not have per-user information
available so it can only provide host-to-host or subnet-to-subnet
protections.
When AH or ESP is used, the application programmer might not need to
do anything (if AH or ESP has been enabled system-wide) or might need
to make specific software changes (e.g. adding specific setsockopt()
calls) -- depending on the AH or ESP implementation being used.
The primary difficulty with IPsec is that it is not widely deployed
at present. In particular, initial IPsec deployment has largely been
in security gateways rather than in hosts. In turn, this means that
fine-grained identity information (e.g. user identification) is not
yet practical to use in the global Internet. Because AH and ESP are
integrated with the Internet Protocol implementation, adding support
for AH or ESP generally means a new operating system version or new
kernel version needs to be deployed. This is a more daunting under-
taking than simply adding some application to an existing system, so
it will take some time for host implementations of AH or ESP to
become widespread.
4.3.2. SSL/TLS
The currently most common approach is to use SSL or it's successor
TLS. They provide channel security for a TCP connection at the
Rescorla, Korver [Page 13] Internet-Draft Security Considerations Guidelines
application level. That is, they run over TCP. SSL implementations
typically provide a Berkeley Sockets-like interface for easy program-
ming. The primary issue when designing a protocol solution around
TLS is to differentiate between connections protected using TLS and
those which are not. Note that TLS will not operate over UDP.
The two primary approaches used are to have a separate well-known
port for TLS connections (e.g. the HTTP over TLS port is 443) [REF]
or to have a mechanism for negotiating upward from the base protocol
to TLS. [REF: SMTP/TLS, HTTP Upgrade] When an upward negotiation
strategy is used, care must be taken to ensure that an attacker can
not force a clear connection when both parties wish to use TLS.
4.3.3. Remote Login
In some special cases it may be worth providing channel-level secu-
rity directly in the application rather than using IPSEC or SSL/TLS.
One such case is remote terminal security. Characters are typically
delivered from client to server one character at a time. Since
SSL/TLS and AH/ESP MAC and encrypt every packet, this can mean a data
expansion of 20-fold. The telnet encryption option [REF] prevents
this expansion by foregoing message integrity.
When using remote terminal service, it's often desirable to securely
perform other sorts of communications services. In addition to pro-
viding remote login, SSH [REF] also provides secure port forwarding
for arbitrary TCP ports, thus allowing users run arbitrary TCP-based
applications over the SSH channel. Note that this capability also
represents a security vulnerability in that it circumvents firewalls
and may potentially expose insecure applications to the outside
world.
4.4. Denial of Service Attacks and Countermeasures
Denial of service attacks are all too frequently viewed as an fact of
life. One problem is that an attacker can often choose from one of
many denial of service attacks to inflict upon a victim, and because
most of these attacks cannot be thwarted, common wisdom frequently
assumes that there is no point protecting against one kind of denial
of service attack when there are many other denial of service attacks
that are possible but that cannot be prevented.
However, not all denial of service attacks are equal and more impor-
tantly, it is possible to design protocols such that denial of ser-
vice attacks are made more difficult if not impractical. Recent SYN
flood attacks [REF??] demonstrate both of these properties: SYN
flood attacks are so easy, anonymous, and effective that they are
more attractive to attackers than other attacks; and because the
Rescorla, Korver [Page 14] Internet-Draft Security Considerations Guidelines
design of TCP enables this attack.
Authors of internet standards MUST describe which denial of service
attacks their protocol is susceptable to. This description MUST
include the reasons it was either unreasonable or out of scope to
attempt to avoid these denial of service attacks.
4.4.1. Blind Denial of Service
BLIND denial of service attacks are particularly pernicious. With a
blind attack the attacker has a significant advantage. If the
attacker must be able to receive traffic from the victim then he must
either subvert the routing fabric or must use his own IP address.
Either provides an opportunity for victim to track the attacker
and/or filter out his traffic. With a blind attack the attacker can
use forged IP addresses, making it extremely difficult for the victim
to filter out his packets. The TCP SYN flood attack is an example of
a blind attack. Designers should make every attempt possible to pre-
vent blind denial of service attacks.
4.4.2. Avoiding Denial of Service
There are two common approaches to making denial of service attacks more
difficult:
4.4.2.1. Make your attacker do more work than you do
If an attacker consumes more of his resources than yours when launch-
ing an attack, attackers with fewer resources than you will be unable
to launch effective attacks. One common technique is to require the
attacker perform a time-intensive operation, such as a cryptographic
operation. Note that an attacker can still mount a denial of service
attack if he can muster substantially sufficient CPU power. For
instance, this technique would not stop the distributed attacks
described in [REF].
4.4.2.2. Make your attacker prove they can receive data from you
A blind attack can be subverted by forcing the attack prove that they
can can receive data from the victim. A common technique is to
require that the attacker reply using information that was gained
earlier in the message exchange. If this countermeasure is used, the
attacker must either use his own address (making him easy to track)
or to forge an address which will be routed back along a path that
traverses the host from which the attack is being launched.
Hosts on small subnets are thus useless to the attacker (at least in
the context of a spoofing attack) because the attack can be traced
back to a subnet (which should be sufficient for locating the
Rescorla, Korver [Page 15] Internet-Draft Security Considerations Guidelines
attacker) so that anti-attack measures can be put into place (for
instance, a boundary router can be configured to drop all traffic
from that subnet). A common technique is to require that the
attacker reply using information that was gained earlier in the mes-
sage exchange.
4.4.3. Example: TCP SYN Floods
TCP/IP is vulnerable to SYN flood attacks (which are described in
section 3.3.2) because of the design of the 3-way handshake. First,
an attacker can force a victim to consume significant resources (in
this case, memory) by sending a single packet. Second, because the
attacker can perform this action without ever having received data
from the victim, the attack can be performed anonymously (and there-
fore using a large number of forged source addresses).
4.4.4. Example: Photuris
Photuris [RFC2522] implements an anti-clogging mechanism that pre-
vents attacks on Photuris that resemble the SYN flood attack. Pho-
turis employs a time-variant secret to generate a "cookie" which is
returned to the attacker. This cookie must be returned in subsequent
messages for the exchange to progress. The interesting feature is
that this cookie can be re-generated by the victim later in the
exchange, and thus no state need be retained by the victim until
after the attacker has proven that he can receive packets from the
victim.
4.5. Object vs. Channel Security
It's useful to make the conceptual distinction between object secu-
rity and channel security. Object security refers to security mea-
sures which apply to entire data objects. Channel security measures
provide a secure channel over which objects may be carried transpar-
ently but the channel has no special knowledge about object bound-
aries.
Consider the case of an email message. When it's carried over an
IPSEC or TLS secured connection, the message is protected during
transmission. However, it is unprotected in the receiver's mailbox,
and in intermediate spool files along the way.
By contrast, when an email message is protected with S/MIME or
OpenPGP, the entire message is encrypted and integrity protected
until it is examined and decrypted by the recipient. It also pro-
vides strong authentication of the actual sender, as opposed to the
machine the message came from. This is object security. Moreover,
the receiver can prove the signed message's authenticity to a third
Rescorla, Korver [Page 16] Internet-Draft Security Considerations Guidelines
party.
Note that the difference between object and channel security is a
matter of perspective. Object security at one layer of the protocol
stack often looks like channel security at the next layer up. So,
from the perspective of the IP layer, each packet looks like an indi-
vidually secured object. But from the perspective of a web client,
IPSEC just provides a secure channel.
The distinction isn't always clear-cut. For example, S-HTTP provides
object level security for a single HTTP transaction, but a web page
typically consists of multiple HTTP transactions (the base page and
numerous inline images.) Thus, from the perspective of the total web
page, this looks rather more like channel security. Object security
for a web page would consist of security for the transitive closure
of the page and all its embedded content as a single unit.
4.6. Defending Aganst Denial of Service
5. Writing Security Considerations Sections
While it is not a requirement that any given protocol or system be
immune to all forms of attack, it is still necessary for authors to
consider them. Part of the purpose of the Security Considerations
section is to explain what attacks are out of scope and what counter-
measures can be applied to defend against them.
There should be a clear description of the kinds of threats on the
described protocol or technology. This should be approached as an
effort to perform "due diligence" in describing all known or foresee-
able risks and threats to potential implementers and users.
Authors MUST describe
1. which attacks are out of scope (and why!)
2. which attacks are in-scope
2.1 and the protocol is susceptable to
2.2 and the protocol protects against
At least the following forms of attack MUST be considered: eavesdrop-
ping, replay, message insertion, deletion, modification, and man-in-
the-middle. Potential denial of service attacks MUST be identified as
well. If the protocol incorporates cryptographic protection mecha-
nisms, it should be clearly indicated which portions of the data are
protected and what the protections are (i.e. integrity only,
Rescorla, Korver [Page 17] Internet-Draft Security Considerations Guidelines
confidentiality, and/or endpoint authentication, etc.). Some indica-
tion should also be given to what sorts of attacks the cryptographic
protection is susceptible. Data which should be held secret (keying
material, random seeds, etc.) should be clearly labeled.
If the technology involves authentication, particularly, user-host
authentication, the security of the authentication method MUST be
clearly specified. That is, authors MUST document the assumptions
that the security of this authentication method is predicated upon.
For instance, in the case of the UNIX username/password login method,
a statement to the effect of:
Authentication in the system is secure only to the extent that it
is difficult to guess or obtain a ASCII password that is a maximum
of 8 characters long. These passwords can be obtained by sniffing
telnet sessions or by running the 'crack' program using the con-
tents of the /etc/passwd file. Attempts to protect against on-line
password guessing by (1) disconnecting after several unsuccessful
login attempts and (2) waiting between successive password prompts
is effective only to the extent that attackers are impatient.
Because the /etc/passwd file maps usernames to user ids, groups,
etc. it must be world readable. In order to permit this usage but
make running crack more difficult, the file is often split into
/etc/passwd and a 'shadow' password file. The shadow file is not
world readable and contains the encrypted password. The regular
/etc/passwd file contains a dummy password in its place.
It is insufficient to simply state that one's protocol should be run
over some lower layer security protocol. If a system relies upon
lower layer security services for security, the protections those
services are expected to provide MUST be clearly specified. In addi-
tion, the resultant properties of the combined system need to be
specified.
Note: In general, the IESG will not approve standards track protocols
which do not provide for strong authentication, either internal to
the protocol or through tight binding to a lower layer security pro-
tocol.
The threat environment addressed by the Security Considerations sec-
tion MUST at a minimum include deployment across the global Internet
across multiple administrative boundaries without assuming that fire-
walls are in place, even if only to provide justification for why
such consideration is out of scope for the protocool. It is not
acceptable to only discuss threats applicable to LANs and ignore the
broader threat environment. All IETF standards-track protocols are
considered likely to have deployment in the global Internet. In some
Rescorla, Korver [Page 18] Internet-Draft Security Considerations Guidelines
cases, there might be an Applicability Statement discouraging use of
a technology or protocol in a particular environment. Nonetheless,
the security issues of broader deployment should be discussed in the
document.
There should be a clear description of the residual risk to the user
or operator of that protocol after threat mitigation has been
deployed. Such risks might arise from compromise in a related proto-
col (e.g. IPSEC is useless if key management has been compromised),
from incorrect implementation, compromise of the security technology
used for risk reduction (e.g. 40-bit DES), or there might be risks
that are not addressed by the protocol specification (e.g. denial of
service attacks on an underlying link protocol).
There should also be some discussion of potential security risks
arising from potential misapplications of the protocol or technology
described in the RFC. This might be coupled with an Applicability
Statement for that RFC.
6. Examples
This section consists of some example security considerations sec-
tions, intended to give the reader a flavor of what's intended by
this document.
The first example is a 'retrospective' example, applying the criteria
of this document to a historical document, RFC-821. The second exam-
ple is a good security considerations section clipped from a current
protocol.
6.1. SMTP
When RFC-821 was written, Security Considerations sections were not
required in RFCs, and none is contained in that document. Had that
document been written today, the Security Considerations section
might look something like this:
6.1.1. SMTP Security Considerations
SMTP as-is provides no security precautions of any kind. All the
attacks we are about to describe must be provided by a different pro-
tocol layer.
A passive attack is sufficient to recover message text. No endpoint
authentication is provided by the protocol. Sender spoofing is triv-
ial, and therefore forging email messages is trivial. Some implemen-
tations do add header lines with hostnames derived through reverse
name resolution (which is only secure to the extent that it is
Rescorla, Korver [Page 19] Internet-Draft Security Considerations Guidelines
difficult to spoof DNS -- not very), although these header lines are
normally not displayed to users. Receiver spoofing is also fairly
straight-forward, either using TCP connection hijacking or DNS spoof-
ing. Moreover, since email messages often pass through SMTP gateways,
all intermediate gateways must be trusted, a condition nearly impos-
sible on the global Internet.
Several approaches are available for alleviating these threats. In
order of increasingly high level in the protocol stack, we have:
SMTP over IPSEC
SMTP/TLS
S/MIME and PGP/MIME
6.1.1.1. SMTP over IPSEC
An SMTP connection run over IPSEC can provide confidentiality for the
message between the sender and the first hop SMTP gateway, or between
any pair of connected SMTP gateways. That is to say, it provides
channel security for the SMTP connections. In a situation where the
message goes directly from the client to the receiver's gateway, this
may provide substantial security (though the receiver must still
trust the gateway). Protection is provided against replay attacks,
since the data itself is protected and the packets cannot be
replayed.
Endpoint identification is a problem, however, unless the receiver's
address can be directly cryptographically authenticated. No sender
identification is available, since the sender's machine is authenti-
cated, not the sender himself. Furthermore, the identity of the
sender simply appears in the From header of the message, so it is
easily spoofable by the sender. Finally, unless the security policy
is set extremely strictly, there is also an active downgrade to
cleartext attack.
6.1.1.2. SMTP/TLS
SMTP can be combined with TLS as described in [SMTPTLS]. This pro-
vides similar protection to that provided when using IPSEC. Since
TLS certificates typically contain the server's host name, recipient
authentication may be slightly more obvious, but is still susceptible
to DNS spoofing attacks. Notably, common implementations of TLS con-
tain a US exportable (and hence low security) mode. Applications
desiring high security should ensure that this mode is disabled.
Protection is provided against replay attacks, since the data itself
is protected and the packets cannot be replayed. [note: The Security
Considerations section of the SMTP over TLS draft is quite good and
Rescorla, Korver [Page 20] Internet-Draft Security Considerations Guidelines
bears reading as an example of how to do things.]
6.1.1.3. S/MIME and PGP/MIME
S/MIME and PGP/MIME are both message oriented security protocols.
They provide object security for individual messages. With various
settings, sender and recipient authentication and confidentiality may
be provided. More importantly, the identification is not of the send-
ing and receiving machines, but rather of the sender and recipient
themselves. (Or, at least, of cryptographic keys corresponding to the
sender and recipient.) Consequently, end-to-end security may be
obtained. Note, however, that no protection is provided against
replay attacks.
6.1.1.4. Denial of Service
None of these security measures provides any real protection against
denial of service. SMTP connections can easily be used to tie up sys-
tem resources in a number of ways, including excessive port consump-
tion, excessive disk usage (email is typically delivered to disk
files), and excessive memory consumption (sendmail, for instance, is
fairly large, and typically forks a new process to deal with each
message.)
6.1.1.5. Inappropriate Usage
In particular, there is no protection provided against unsolicited
mass email (aka SPAM).
SMTP also includes several commands which may be used by attackers to
explore the machine on which the SMTP server runs. The VRFY command
permits an attacker to convert user-names to mailbox name and often
real name. This is often useful in mounting a password guessing
attack, as many users use their name as their password. EXPN permits
an attacker to expand an email list to the names of the subscribers.
This may be used in order to generate a list of legitimate users in
order to attack their accounts, as well as to build mailing lists for
future SPAM. Administrators may choose to disable these commands.
6.2. VRRP
The second example is from VRRP, the Virtual Router Redundance Proto-
col ([RFC2388]). We reproduce here the Security Considerations sec-
tion from that document (with new section numbers). Our comments are
indented and prefaced with 'NOTE:'.
Rescorla, Korver [Page 21] Internet-Draft Security Considerations Guidelines
6.2.1. Security Considerations
VRRP is designed for a range of internetworking environments that may
employ different security policies. The protocol includes several
authentication methods ranging from no authentication, simple clear
text passwords, and strong authentication using IP Authentication
with MD5 HMAC. The details on each approach including possible
attacks and recommended environments follows.
Independent of any authentication type VRRP includes a mechanism
(setting TTL=255, checking on receipt) that protects against VRRP
packets being injected from another remote network. This limits most
vulnerabilities to local attacks.
NOTE: The security measures discussed in the following sections
only provide various kinds of authentication. No confidentiality
is provided at all. This should be explicitly described as outside
the scope.
6.2.1.1. No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. This type of authentication SHOULD
only be used in environments were there is minimal security risk and
little chance for configuration errors (e.g., two VRRP routers on a
LAN).
6.2.1.2. Simple Text Password
The use of this authentication type means that VRRP protocol
exchanges are authenticated by a simple clear text password.
This type of authentication is useful to protect against accidental
misconfiguration of routers on a LAN. It protects against routers
inadvertently backing up another router. A new router must first be
configured with the correct password before it can run VRRP with
another router. This type of authentication does not protect against
hostile attacks where the password can be learned by a node snooping
VRRP packets on the LAN. The Simple Text Authentication combined
with the TTL check makes it difficult for a VRRP packet to be sent
from another LAN to disrupt VRRP operation.
This type of authentication is RECOMMENDED when there is minimal risk
of nodes on a LAN actively disrupting VRRP operation. If this type
of authentication is used the user should be aware that this clear
text password is sent frequently, and therefore should not be the
same as any security significant password.
Rescorla, Korver [Page 22] Internet-Draft Security Considerations Guidelines
6.2.1.3. IP Authentication Header
The use of this authentication type means the VRRP protocol exchanges
are authenticated using the mechanisms defined by the IP Authentica-
tion Header [AUTH] using "The Use of HMAC-MD5-96 within ESP and AH",
[HMAC]. This provides strong protection against configuration
errors, replay attacks, and packet corruption/modification.
This type of authentication is RECOMMENDED when there is limited con-
trol over the administration of nodes on a LAN. While this type of
authentication does protect the operation of VRRP, there are other
types of attacks that may be employed on shared media links (e.g.,
generation of bogus ARP replies) which are independent from VRRP and
are not protected.
NOTE: Specifically, although securing VRRP prevents unauthorized machines
from taking part in the election protocol, it does not protect
hosts on the network from being deceived. For example, a gratutitous
ARP reply from what purports to be the virtual router's IP address
can redirect traffic to an unauthorized machine. Similarly,
individual connections can be diverted by means of forged ICMP
Redirect messages.
Acknowledgments
This document is heavily based on a note written by Ran Atkinson in
1997. That note was written after the IAB Security Workshop held in
early 1997, based on input from everyone at that workshop. Some of
the specific text above was taken from Ran's original document, and
some of that text was taken from an email message written by Fred
Baker.
The other primary source for this document is specific comments
received from Steve Bellovin. Early review of this document was done
by Lisa Dusseault and Mark Schertler.
References
[CA-96.21] "TCP SYN Flooding and IP Spoofing", CERT Advisory CA-96.21
ftp://info.cert.org/pub/cert advisories/CA-96.21.tcp syn flooding
[RFC 1704]
The rest are TBD
Security Considerations
This entire document is about security considerations.
Rescorla, Korver [Page 23] Internet-Draft Security Considerations Guidelines
Author's Address
Eric Rescorla <ekr@rtfm.com>
RTFM, Inc.
30 Newell Road #16
East Palo Alto, CA 94303
Phone: (650) 328-8631
Brian Korver <briank@network-alchemy.com>
Network Alchemy
1538 Pacific Avenue
Santa Cruz, CA 95060
Phone: (831)-460-3800
Rescorla, Korver [Page 24] Internet-Draft Security Considerations Guidelines
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. The Goals of Security . . . . . . . . . . . . . . . . . . . . . . 1
2.1. Communications Security . . . . . . . . . . . . . . . . . . . . 2
2.1.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.2. Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.3. Endpoint authentication . . . . . . . . . . . . . . . . . . . 3
2.2. Systems Security . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.1. Unauthorized Usage . . . . . . . . . . . . . . . . . . . . . 3
2.2.2. Inappropriate Usage . . . . . . . . . . . . . . . . . . . . . 4
2.2.3. Denial of Service . . . . . . . . . . . . . . . . . . . . . . 4
3. The Internet Threat Model . . . . . . . . . . . . . . . . . . . . 4
3.1. Limited Threat Models . . . . . . . . . . . . . . . . . . . . . 5
3.2. Passive Attacks . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Privacy Violations . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. Password Sniffing . . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. Offline Cryptographic Attacks . . . . . . . . . . . . . . . . 6
3.3. Active Attacks . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.1. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.2. Message Insertion . . . . . . . . . . . . . . . . . . . . . . 8
3.3.3. Message Deletion . . . . . . . . . . . . . . . . . . . . . . 8
3.3.4. Message Modification . . . . . . . . . . . . . . . . . . . . 9
3.3.5. Man-In-The-Middle . . . . . . . . . . . . . . . . . . . . . . 9
4. Common Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. User Authentication . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Username/Password . . . . . . . . . . . . . . . . . . . . . . 10
4.1.2. Challenge Response and One Time Passwords . . . . . . . . . . 10
4.1.3. Certificates . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.4. Some Uncommon Systems . . . . . . . . . . . . . . . . . . . . 11
4.1.5. Host Authentication . . . . . . . . . . . . . . . . . . . . . 11
4.2. Authorization vs. Authentication . . . . . . . . . . . . . . . 12
4.2.1. Access Control Lists . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Certificate Based Systems . . . . . . . . . . . . . . . . . . 12
4.3. Providing Traffic Security . . . . . . . . . . . . . . . . . . 13
4.3.1. IPSEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3.2. SSL/TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3.3. Remote Login . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Denial of Service Attacks and Countermeasures . . . . . . . . . 14
4.4.1. Blind Denial of Service . . . . . . . . . . . . . . . . . . . 15
4.4.2. Avoiding Denial of Service . . . . . . . . . . . . . . . . . 15
4.4.2.1. Make your attacker do more work than you do . . . . . . . . 15
4.4.2.2. Make your attacker prove they can receive data from you
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4.3. Example: TCP SYN Floods . . . . . . . . . . . . . . . . . . . 16
4.4.4. Example: Photuris . . . . . . . . . . . . . . . . . . . . . . 16
4.5. Object vs. Channel Security . . . . . . . . . . . . . . . . . . 16
Rescorla, Korver [Page 25] Internet-Draft Security Considerations Guidelines
4.6. Defending Aganst Denial of Service . . . . . . . . . . . . . . 17
5. Writing Security Considerations Sections . . . . . . . . . . . . 17
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. SMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1.1. SMTP Security Considerations . . . . . . . . . . . . . . . . 19
6.1.1.1. SMTP over IPSEC . . . . . . . . . . . . . . . . . . . . . . 20
6.1.1.2. SMTP/TLS . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1.1.3. S/MIME and PGP/MIME . . . . . . . . . . . . . . . . . . . . 21
6.1.1.4. Denial of Service . . . . . . . . . . . . . . . . . . . . . 21
6.1.1.5. Inappropriate Usage . . . . . . . . . . . . . . . . . . . . 21
6.2. VRRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2.1. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6.2.1.1. No Authentication . . . . . . . . . . . . . . . . . . . . . 22
6.2.1.2. Simple Text Password . . . . . . . . . . . . . . . . . . . 22
6.2.1.3. IP Authentication Header . . . . . . . . . . . . . . . . . 23
6.2.1.3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 23
6.2.1.3. References . . . . . . . . . . . . . . . . . . . . . . . . 23
Security Considerations . . . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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