CAPEC-42: MIME Conversion
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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Overview
CAPEC-42 (MIME Conversion) is a detailed-level attack pattern catalogued by MITRE in the Common Attack Pattern Enumeration and Classification (CAPEC). It describes a recurring method attackers use to exploit software weaknesses.
How the attack works
The phases an attacker typically follows to carry out this attack.
- Step 1Explore
[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.
- Step 2Explore
[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).
- Step 3Experiment
[Find injection vector] Identify places in the system where vulnerable MIME conversion routines may be used.
- Step 4Experiment
[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
- Step 4Exploit
[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.
What the attacker needs
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 CAPEC-42 attack can achieve.
Execute Unauthorized Commands
Affects: Confidentiality, Integrity, Availability
Run Arbitrary Code
Modify Data
Affects: Integrity
Unreliable Execution
Affects: Availability
Gain Privileges
Affects: Confidentiality, Access Control, Authorization
How to mitigate it
Defenses that reduce the risk of CAPEC-42.
- 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
Examples
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
Frequently asked questions
Common questions about CAPEC-42.
- What is CAPEC-42?
- 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.
- How does a MIME Conversion attack work?
- It typically unfolds over 5 phases. It begins with: [Identify target mail server] The adversary identifies a target mail server that they wish to attack.
- How do you prevent CAPEC-42?
- Stay up to date with third party vendor patches
- What weaknesses does CAPEC-42 target?
- CAPEC-42 exploits 4 CWE weaknesses, including CWE-20 (Improper Input Validation), CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')), CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer), CWE-120 (Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')).
- How severe is CAPEC-42?
- MITRE rates CAPEC-42 as High severity with high likelihood of attack.
References
Attack-pattern data is sourced from the MITRE CAPEC catalog (v3.9). Weakness associations link to the corresponding CWE entries on RadicalNotion.AI.
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