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CAPEC-42: MIME Conversion

Attack Pattern ID: 42
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
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.
+ Likelihood Of Attack


+ Typical Severity


+ Relationships
Section HelpThis table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.100Overflow Buffers
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
+ Execution Flow
  1. Identify target mail server: The adversary identifies a target mail server that they wish to attack.

    Use Nmap on a system to identify a mail server service.
  2. Determine viability of attack: 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).

  1. Find injection vector: Identify places in the system where vulnerable MIME conversion routines may be used.

  2. Craft overflow content: The adversary crafts e-mail messages with special headers that will cause a buffer overflow for the vulnerable MIME conversion routine. The intent of this attack is to leverage the overflow for execution of arbitrary code and gain access to the mail server machine, so the adversary will craft an email that not only overflows the targeted buffer but does so in such a way that the overwritten return address is replaced with one of the adversary's choosing.

    Create malicious shellcode that will execute when the program execution is returned to it.
    Use a NOP-sled in the overflow content to more easily "slide" into the malicious code. This is done so that the exact return address need not be correct, only in the range of all of the NOPs
  1. Overflow the buffer: Send e-mail messages to the target system with specially crafted headers that trigger the buffer overflow and execute the shell code.

+ 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.
+ Skills Required
[Level: Low]
It may be trivial to cause a DoS via this attack pattern
[Level: High]
Causing arbitrary code to execute on the target system.
+ Consequences
Section HelpThis table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Execute Unauthorized Commands
Modify Data
Unreliable Execution
Access Control
Gain Privileges
+ 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 file.

For example, a 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,
A=mail -d $u

Mprog, P=/bin/sh, F=lsDFMoqeu, S=10/30, R=20/40,
A=sh -c $u

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

`translit(LOCAL_MAILER_FLAGS, `9')',

`translit(LOCAL_SHELL_FLAGS, `9')',

and then rebuilding the file using m4(1).

From "Exploiting Software", please see reference below.

Use the sendmail restricted shell program (smrsh)
Use mail.local
+ Example Instances
Attack Example: Sendmail Overflow

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.

See also: CVE-1999-0047
+ References
[REF-1] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
[REF-364] "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. <>.
+ Content History
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modification DateModifierOrganization
2021-10-21CAPEC Content TeamThe MITRE Corporation
Updated Execution_Flow
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Page Last Updated or Reviewed: October 21, 2021