Common Attack Pattern Enumeration and Classification
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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
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.
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.
The first step is to figure what mail server (and what version) is running on the target system.
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,
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:
and then rebuilding the sendmail.cf file using m4(1).
From "Exploiting Software", please see reference below.
Use the sendmail restricted shell program (smrsh)
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.
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.
[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>.