Tracking down how hackers gain access through web apps

Hackers commonly use vulnerabilities in web applications to gain access to a server. Sometimes, though, it can be difficult to track down exactly how they gained access to a server. Especially if the server hosts a bunch of websites and there are lots of potentially vulnerable scripts.

I’ve tracked down more of these than I can count, and have sortof developed a pattern for investigating. Here are some useful things to try:

1- Look in /tmp and /var/tmp for possibly malicious files. These directories are usually world-writable, and commonly used to temporarily store files. Sometimes the files are disguised with leading dot’s, or they may be named something that looks similar to other files in the directory like “. ” (dot- space), or like a session files named sess_something.

If you are able to see any files, you can use the timestamps of the files to try and look through some Apache logs to find the exact hit that it came from

2- If a rogue process is still running, look at the /proc entry for that file to determine more information about it. The files in /proc/<PID> will tell you information like the executable file that created the process, it’s working directory, environment information, and plenty more details. Usually, the rogue processes are running as the apache user (httpd, nobody, apache).

If all of the rogue processes were being run by the Apace user, then the hacker likely didn’t gain root access. If you have rogue processes that were being run by root, it is much harder to clean up after. Usually the only truly safe method is to start over with a clean installation.

3- netstat -l will help you identify processes that are listening for incoming connections. Often times, these are a perl script. Sometimes they are named things that look legitmiate like ‘httpd’, so pay close attention. netstat-n will help you to see current connections that your server has to others.

4- Look in your error logs for files being downloaded with wget. A common tactic is for hackers to run a wget command to download another file with more malicious instructions. Fortunately, wget writes to STDERR, so it’s output is usually displayed in the error logs. Something like this is evidence of a successful wget:

--20:30:40--  http://somehackedsite.com/badfile.txt
            => `Lnx.txt'
Resolving somehackedsite.com... 12.34.56.78

Connecting to somehackedsite.com[12.34.56.78]:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 12,345 [text/plain]

     0K .......... ......                                     100%  263.54 KB/s

20:30:50 (263.54 KB/s) - `badfile.txt' saved [12,345/12,345]

You can use this information to try and recreate what the hacker did. Look for the file they downloaded (badfile.txt in this case) and look at what it does. You can also used these timestamps to look through access_logs to find the vulnerable script.

Since wget is a commonly used tool for this, I like to create a .wgetrc file that contains bogus proxy information, so that even if a hacker is able to attempt a download, it won’t work. Create a .wgetrc file in Apache’s home directory with this content:

http_proxy = http://bogus.dontresolveme.com:19999/
ftp_proxy = http://bogus.dontresolveme.com:19999/

5- If you were able to identify any timestamps, you can grep through Apache logs to find requests from that time. If you have a well-structured server where you have logs in a consistent place, then you can use a command like this to search all of the log files at onces:

grep "01\\/Jun\\/2007:10:20:" /home/*/logs/access_log

I usually leave out the seconds field because requests sometimes take several seconds to execute. If you have a server name or file name that you found was used by a wget, you can try searching for those too:

grep "somehackesite.com" /home/*/logs/access_log

6 – Turn of PHP’s register_globals by default and only enable it if truly needed. If you write PHP apps, learn how to program securely, and never rely on register_globals being on.

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