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CAPEC-42: MIME Conversion

 
MIME Conversion
Definition in a New Window Definition in a New Window
Attack Pattern ID: 42
Abstraction: Detailed
Status: Draft
Completeness: Complete
+ Description

Summary

An attacker exploits a weakness in the MIME conversion routine to cause a buffer overflow and gain control over the mail server machine. The MIME system is designed to allow various different information formats to be interpreted and sent via e-mail. Attack points exist when data are converted to MIME compatible format and back.

Attack Execution Flow

Explore
  1. Determine whether the mail server is unpatched and is potentially vulnerable to one of the known MIME conversion buffer overflows (e.g. Sendmail 8.8.3 and 8.8.4).

  2. Identify places in the system where vulnerable MIME conversion routines may be used.

Exploit
  1. Send e-mail messages to the target system with specially crafted headers that trigger the buffer overflow and execute the shell code.

+ Attack Prerequisites
  • The target system uses a mail server.

  • Mail server vendor has not released a patch for the MIME conversion routine, the patch itself has a security hole or does not fix the original problem, or the patch has not been applied to the user's system.

+ Typical Severity

High

+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-Instances

Description

A MIME conversion buffer overflow exists in Sendmail versions 8.8.3 and 8.8.4. Sendmail versions 8.8.3 and 8.8.4 are vulnerable to a buffer overflow in the MIME handling code. By sending a message with specially-crafted headers to the server, a remote attacker can overflow a buffer and execute arbitrary commands on the system with root privileges.

Sendmail performs a 7 bit to 8 bit conversion on email messages. This vulnerability is due to the fact that insufficient bounds checking was performed while performing these conversions. This gave attacker an opportunity to overwrite the internal stack of sendmail while it is executing with root privileges. An attacker first probes the target system to figure out what mail server is used on the system and what version. An attacker could then test out the exploit at their leisure on their own machine running the same version of the mail server before using it in the wild.

Related Vulnerabilities

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

It may be trivial to cause a DoS via this attack pattern

Skill or Knowledge Level: High

Causing arbitrary code to execute on the target system.

+ Probing Techniques

Description

The first step is to figure what mail server (and what version) is running on the target system.

+ Solutions and Mitigations

Stay up to date with third party vendor patches

Disable the 7 to 8 bit conversion. This can be done by removing the F=9 flag from all Mailer specifications in the sendmail.cf file.

For example, a sendmail.cf file with these changes applied should look similar to (depending on your system and configuration):

Mlocal, P=/usr/libexec/mail.local, F=lsDFMAw5:/|@qrmn, S=10/30, R=20/40,
T=DNS/RFC822/X-Unix,
A=mail -d $u
Mprog, P=/bin/sh, F=lsDFMoqeu, S=10/30, R=20/40,
D=$z:/,
T=X-Unix,
A=sh -c $u

This can be achieved for the "Mlocal" and "Mprog" Mailers by modifying the ".mc" file to include the following lines:

define(`LOCAL_MAILER_FLAGS',
ifdef(`LOCAL_MAILER_FLAGS',
`translit(LOCAL_MAILER_FLAGS, `9')',
`rmn'))
define(`LOCAL_SHELL_FLAGS',
ifdef(`LOCAL_SHELL_FLAGS',
`translit(LOCAL_SHELL_FLAGS, `9')',
`eu'))

and then rebuilding the sendmail.cf file using m4(1).

From "Exploiting Software", please see reference below.

Use the sendmail restricted shell program (smrsh)

Use mail.local

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Confidentiality
Integrity
Availability
Execute unauthorized code or commands
Run Arbitrary Code
Integrity
Modify memory
Availability
DoS: crash / exit / restart
Confidentiality
Access_Control
Authorization
Gain privileges / assume identity
+ Injection Vector

The especially formatted e-mail message whose body is put together in a way as to trigger the MIME conversion buffer overflow in the 7 to 8 bit MIME conversion function.

+ Payload

The shell code included as part of the e-mail message body that is executed on the target system with root privileges after the stack based buffer overflow in the 7 to 8 bit MIME conversion function is leveraged.

+ Activation Zone

The function performing 7 to 8 bit MIME conversion.

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: High
+ Technical Context
Architectural Paradigms
All
Frameworks
All
Platforms
All
Languages
All
+ References
[R.42.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
[R.42.2] "CERT Advisory CA-1997-05 MIME Conversion Buffer Overflow in Sendmail Versions 8.8.3 and 8.8.4". Software Engineering Institute: Carnegie Mellon University. <http://www.cert.org/advisories/CA-1997-05.html>.
+ Content History
Submissions
SubmitterOrganizationDate
[R.42.1][REF-2] Cigital, Inc2007-03-01
Modifications
ModifierOrganizationDateCommentsSource
Eugene LebanidzeCigital, Inc2007-02-26Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-05Review and revise
CAPEC Content TeamThe MITRE Corporation2014-02-06Updated Attack_Phases, Injection_VectorInternal

Page Last Updated: May 07, 2014