Corsaire Security Advisory - Multiple vendor MIME RFC822 comment issue
From: advisories (advisories_at_corsaire.com)
To: <email@example.com> Date: Tue, 14 Sep 2004 16:00:12 +0100
-- Corsaire Security Advisory --
Title: Multiple vendor MIME RFC822 comment issue
Author: Martin O'Neal [firstname.lastname@example.org]
Audience: General distribution
-- Scope --
The aim of this document is to clearly define a MIME content evasion
issue that affects a variety of products including; browsers, proxy
servers, email clients, content security gateways and antivirus
-- History --
Discovered: 04.08.03 (Martin O'Neal)
NISCC notified: 19.02.04
Document released: 13.09.04
-- Overview --
There are a number of content security gateway and antivirus products
available that provide policy based security functionality. Part of this
functionality allows the products to block embedded file attachments
based on their specific content type, such as executables or those
containing viruses. However, by using malformed MIME encapsulation
techniques centred on the presence of fields containing an RFC822
comment, this functionality can be evaded.
-- Analysis --
The MIME standards are intended to provide a common mechanism to
exchange data between systems and are used extensively by protocols such
as HTTP and SMTP. The structure of a MIME message is defined in RFC2045
, which in turn makes use of concepts introduced in RFC822 
(superseded by RFC2822 ).
The standards define a range of fields that control how data is encoded
within the transport, and how it should be interpreted by the receiving
agent. RFC822  states "A comment is a set of ASCII characters, which
is enclosed in matching parentheses and which is not within a quoted-
string. The comment construct permits message originators to add text
which will be useful for human readers, but which will be ignored by the
formal semantics. Comments should be retained while the message is
subject to interpretation according to this standard. However, comments
must NOT be included in other cases, such as during protocol exchanges
with mail servers".
The implementation of the commenting standard has not been universal by
all of the vendors. For many products, such as email clients and
browsers, this scope for variation might only result in some unreliable
behaviour. However, for a collection of security products, being unaware
of the various ways that the standard has been implemented can lead to
more serious results, as the products may fail to detect a threat within
the data stream.
When a receiving agent is presented with a MIME message that contains an
unexpected RFC822 comment, it tends to respond in one of three broad
- It identifies the MIME message as malformed and blocks it.
- It fails to interpret the MIME field (or message).
- It correctly interprets the MIME field (or message).
The first of the three would be the correct behaviour for a security
conscious product, but based on empirical research this is not the
common result for a number of scenarios.
The RFC822 comment issue has been observed to affect many of the
security products. To use this issue as an attack vector, all that is
required is to identify a target that has a client agent that
successfully interprets the RFC822 comment correctly, where any security
products that protect it do not.
-- Recommendations --
To be effective tools, the security products must not only be able to
process encoding techniques implemented as per the relevant standard,
but also common misinterpretations and deliberate corruptions.
As an ongoing process, a study project should be undertaken by the
vendors to identify applications that routinely decode MIME objects and
have a liberal interpretation of the MIME standard.
NISCC have produced a document consolidating a number of vendor
statements on these issues . Contact your vendor directly to
establish whether you are affected by these issues.
-- Background --
This issue was discovered using a custom SMTP/HTTP vulnerability
analysis tool developed by Corsaire's security assessment team. This
tool is not available publicly, but is an example of the specialist
approach used by Corsaire's consultants as part of a commercial security
assessment. To find out more about the cutting edge services provided by
Corsaire simply visit our web site at http://www.corsaire.com
-- CVE --
The Common Vulnerabilities and Exposures (CVE) project has assigned
the name CAN-2004-0162 to this issue. This is a candidate for
inclusion in the CVE list (http://cve.mitre.org), which standardises
names for security problems.
-- References --
-- Revision --
a. Initial release.
b. Added CVE reference.
-- Distribution --
This security advisory may be freely distributed, provided that it
remains unaltered and in its original form.
-- Disclaimer --
The information contained within this advisory is supplied "as-is" with
no warranties or guarantees of fitness of use or otherwise. Corsaire
accepts no responsibility for any damage caused by the use or misuse of
-- About Corsaire --
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in Guildford, Surrey, UK. Corsaire bring innovation, integrity and
analytical rigour to every job, which means fast and dramatic security
performance improvements. Our services centre on the delivery of
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