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FTP Brute Password guessing attacks

Published: 2010-08-27
Last Updated: 2010-08-27 13:22:11 UTC
by Mark Hofman (Version: 1)
4 comment(s)

FTP brute password guessing attacks are a fairly regular occurrence at the moment. The fact that these are occurring with regularity means that they are still working, so If you have an internet facing FTP server then there are a few things that you might consider doing to help weather the storm.

Watch your logs!
It is surprising when you work on an incident to see how long an event goes unnoticed. Sometimes months, even though the logs are full of events such as:

 

09:19:44 211.45.113.143 [2]USER Administrator 331 0
09:19:46 211.45.113.143 [2]PASS - 530 1326
09:19:46 211.45.113.143 [2]USER Administrator 331 0
09:19:46 211.45.113.143 [2]PASS - 530 1326
09:19:46 211.45.113.143 [2]USER Administrator 331 0
09:19:47 211.45.113.143 [2]PASS - 530 1326
09:19:47 211.45.113.143 [2]USER Administrator 331 0
09:19:47 211.45.113.143 [2]PASS - 530 1326
09:19:47 211.45.113.143 [2]USER Administrator 331 0
09:19:48 211.45.113.143 [2]PASS - 530 1326
09:19:48 211.45.113.143 [2]USER Administrator 331 0
09:19:48 211.45.113.143 [2]PASS - 530 1326

 

It is quite clear what is going on here. a user typing a password multiple times per second? not likely. The log shows very clearly what is going on someone is guessing passwords. In this case it was a Microsoft FTP server which was being attacked, so there is likely to be an administrator account on the system and eventually this attack result in access. 

Many people don't have their logging enabled. Make sure it is switched on and watched regularly, this is something junior can do on his own.  

Rename Administrator
On windows systems I like renaming the administrator account and then setting up a new user called Administrator, but without any privileges or access on the system.  I set the password to something very long and then watch the logs.  Even if they eventually manage to guess the password the account is not worth anything.  It is a simple thing to do, but can be very effective.  The FTP brute password attack above won't work and you may pick something else up as well.  Simple but effective.  

Remove Anonymous Access
Should you remove Anonymous access? I guess the answer depends on why there is an FTP server in the first place.  Anonymous access is usually abused.  When placing a FTP honeypot on the network the first files start getting uploaded, usually within the hour.  So unless you really need it, remove it. 

Restrict Access to FTP
In many organisations the actual use of FTP is fairly limited.  There is no need for the whole internet to access the FTP server there may be a finite number of locations.  Restrict access to FTP to these locations only, either through firewall rules, or on the FTP server itself (or even both).  This will limit the opportunity for abuse of your FTP server. 

The above are a few simple ways to reduce the risk of losing your FTP server to someone else. If you have some nifty tricks that will help protect an FTP service, write a comment or use the contact form. 

Cheers
Mark H

 

 

 

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