[Full-Disclosure] Corsaire Security Advisory - Multiple vendor MIME RFC2047 encoding issue

– Corsaire Security Advisory –

Title: Multiple vendor MIME RFC2047 encoding issue
Date: 04.08.03
Application: various
Environment: various
Author: Martin O'Neal [[email protected]]
Audience: General distribution
Reference: c030804-007

– 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
products.

– 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 encoded using the RFC2047
parameter value character set and language information, 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
[1], which in turn makes use of concepts introduced in RFC822 [2]
(superseded by RFC2822 [3]).

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. RFC2047 [4] defines "techniques to allow the encoding of non-
ASCII text in various portions of a RFC 822 [2] message header, in a
manner which is unlikely to confuse existing message handling software".

The implementation of these encoding standards 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
unknown RFC2047 encoding, it tends to respond in one of two broad ways:

• It identifies the MIME message as malformed and blocks it.
• It fails to interpret the MIME field (or message).

The first of the two 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 RFC2047 encoding 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 RFC2047 encoding 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 [5]. 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-2004-0053 to this issue. This is a candidate for
inclusion in the CVE list (http://cve.mitre.org), which standardises
names for security problems.

– References –

[1] http://www.faqs.org/rfcs/rfc2045.html
[2] http://www.faqs.org/rfcs/rfc822.html
[3] http://www.faqs.org/rfcs/rfc2822.html
[4] http://www.faqs.org/rfcs/rfc2047.html
[5] http://www.uniras.gov.uk/vuls/2004/380375/mime.htm

– Revision –

a. Initial release.
b. Included CVE reference.
c. Released.

– 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
this information.

– About Corsaire –

Corsaire are a leading information security consultancy, founded in 1997
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
information security planning, assessment, implementation, management
and vulnerability research.

A free guide to selecting a security assessment supplier is available at
http://www.penetration-testing.com

Copyright 2003 Corsaire Limited. All rights reserved.

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