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Update (July 2010) : the ftp fuzzer has been merged into the Metasploit tree. You do no longer need to download the script here. If you are using the latest version of Metasploit (svn update), then you can find the fuzzer in the modules/auxiliary/fuzzers/ftp folder (file is called ftp_pre_post.rb.

Usage :

Navigate to the metasploit folder, go into ‘modules’ and then into ‘auxiliary’

Create a subfolder called ‘fuzzer’ and place the fuzz_ftp.rb file in that folder

Open Metasploit console and select the module

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|    \ / _  )  _)/ _  |/___)  _ \| |/ _ \| |  _)
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       =[ msf v3.3-dev [core:3.3 api:1.0]
+ -- --=[ 429 exploits - 261 payloads
+ -- --=[ 21 encoders - 8 nops
       =[ 199 aux

msf > use auxiliary/fuzzer/fuzz_ftp
msf auxiliary(fuzz_ftp) > show options

Module options:

   Name          Current Setting     Required  Description
   ----          ---------------     --------  -----------
   DELAY         0.5                 no        Delay between connections
   ENDSIZE       20000               no        Fuzzing string endsize
   PASS          anonymous@test.com  no        Password
   RHOSTS                            yes       The target address range or CIDR identifier
   RPORT         21                  yes       The target port
   STARTATSTAGE  1                   no        Start at this test stage
   STARTSIZE     10                  no        Fuzzing string startsize
   STEPSIZE      10                  no        Increase string size each iteration with this number of chars
   THREADS       1                   yes       The number of concurrent threads
   USER          anonymous           no        Username

msf auxiliary(fuzz_ftp) >

Important options are :

RHOSTS = the ip address of the FTP server

USER and PASS = set to a valid/working Username & Password combination

The fuzzer has 5 stages :

1. Issue no command, only send evil data
2. Fuzz the USER command
3. Fuzz the PASS command (after a valid USER command/login was executed)
4. Fuzz all FTP commands (after a valid login was performed), one command, one fuzz string per session
5. Fuzz all FTP commands (after a valid login was performed), one command with all fuzz combinations for that command per session

You can edit the fuzz data and FTP commands yourself (if you want to) by editing the fuzz_ftp.rb file

Start fuzzing ?  Just issue the ‘run’ command.  When the fuzzer cannot connect anymore, the fuzzing process will be halted, and the last sent string will be displayed.