Execution Flow

Identify target mail server: The adversary identifies a target mail server that they wish to attack.

Techniques
Use Nmap on a system to identify a mail server service.

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).

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

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

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.

MIME Conversion