CAPEC-42

MIME Conversion

High severityHigh likelihoodDetailed pattern

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.

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How the attack unfolds
Prerequisites
Skills required
Consequences
Mitigations
Weaknesses it exploits
Related attack patterns
Examples

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Explain the attack pattern CAPEC-42: MIME Conversion: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/42.html and https://cve.bz/cve/capec/42?via=ask-ai-copy, which lists recent CVEs with the weaknesses it exploits. Cite the sources.

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How the attack unfolds

Explore
The attacker looks for a way in.
1. [Identify target mail server] The adversary identifies a target mail server that they wish to attack.
  1 technique
  - 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).
Experiment
They probe what they found.
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.
  2 techniques
  - 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
Exploit
They carry out the attack.
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

Low skill
It may be trivial to cause a DoS via this attack pattern
High skill
Causing arbitrary code to execute on the target system.

Consequences

What a successful attack can lead to
Impact	Affects	Note
Execute Unauthorized Commands	Confidentiality, Integrity, Availability	Run Arbitrary Code
Modify Data	Integrity
Unreliable Execution	Availability
Gain Privileges	Confidentiality, Access Control, Authorization

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

Weaknesses it exploits

CWE-20 Improper Input Validation
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component
CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer
CWE-120 Buffer Copy without Checking Size of Input

Recent CVEs with these weaknesses

9.6CVE-2026-1081862026-11-10
Improper Input Validation in Zoom Workplace for Windows may allow an unauthenticated user to conduct an escalation of privilege via network access.
4.9CVE-2026-1054272026-11-10
Improper input validation in Ivanti EPMM before version 12.10.0.1, 12.9.0.3, and 12.8.0.5 allows a remote authenticated attacker with admin privileges to permanently disable all Apache HTTPS services resulting in denial of service.
6.7CVE-2026-1054262026-11-10
Improper input validation in Ivanti EPMM before version 12.10.0.1, 12.9.0.3, and 12.8.0.5 allows a remote authenticated attacker with admin privileges to set arbitrary Linux kernel parameters as root.
4.3CVE-2026-1231782026-11-10
An insufficient input validation vulnerability in the listed NETGEAR Orbi Models router allows an authenticated administrator connected to the local network to execute unauthorized commands and modify the router’s software and functionalit…
8.8CVE-2026-1249482026-11-10
Incorrect boundary conditions, use-after-free in the Graphics component. This vulnerability was fixed in Firefox 160 and Firefox ESR 153.7.
8.8CVE-2026-1249472026-11-10
Incorrect boundary conditions in the Graphics component. This vulnerability was fixed in Firefox 160.
8.8CVE-2026-1249262026-11-10
Incorrect boundary conditions in the WebRTC component. This vulnerability was fixed in Firefox 160 and Firefox ESR 153.7.
8.8CVE-2026-1249242026-11-10
Incorrect boundary conditions in the Graphics: WebRender component. This vulnerability was fixed in Firefox 160 and Firefox ESR 153.7.

The newest records naming one of them. Each weakness's page lists them all.

Part of: CAPEC-100 Overflow Buffers

Examples

A MIME conversion buffer overflow exists in Sendmail versions 8.8.3 and 8.8.4. Sendmail versions 8.8.3 and 8.…

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.