06 March 2009 | 4,475 views

fzem – MUA (Mail User Agent) / Mail Client Fuzzer

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fzem is a MUA (mail user agent) fuzzer that fuzzes MAIL/MIME email headers as well as how clients handle SMTP, POP and IMAP responses.


fzem’s purpose is to fuzz MUAs as they process email content and handle server reponses.

How does it work?

fzem has the three main mail protocols implemented as well as mail/mime headers. Using these we can manipulate emails and responses to include fuzzing data from our fuzzing oracle or from the user.

A user can command fzem to run in various modes and perform differently in those modes.

  • Fuzz SMTP OK responses
  • Fuzz SMTP ERROR responses
  • Fuzz SMTP using custom fuzz data
  • Fuzz MAIL/MIME headers
  • Fuzz MAIL/MIME headers using custom fuzz data
  • Fuzz MAIL/MIME headers using custom headers
  • Fuzz POP3 OK responses
  • Fuzz POP3 ERROR responses
  • Fuzz POP3 using custom fuzz data
  • Fuzz IMAP4 OK responses
  • Fuzz IMAP4 ERROR responses
  • Fuzz IMAP4 using custom fuzz data

We can also choose a specific port to listen on (-P) and take advantage of the useful debug option (-D).

You can download fzem here:


Or read more here.


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One Response to “fzem – MUA (Mail User Agent) / Mail Client Fuzzer”

  1. navin 7 March 2009 at 10:59 am Permalink

    wow, and I thought this couldn’t be done!! Thanks krakowlabs!!