How to find spam script location with exim

In this guide, we will teach you how to use the Exim mail log on your VPS or a dedicated server to find possible attempts from spammers to use your scripts, or their own in order to relay spam from your server.

How does spam get sent from my server?

You might have a “tell a friend” feature on your website or another email alerting system on your site. If you’re not careful these can sometimes be exploited by bots for spamming purposes. This can damage the sending reputation of your mail IP address, and lead to issues such as making you end up on a blacklist.

How do I stop spam coming from my server?

Exim, or the MTA (Mail Transfer Agent) on your server handles email deliveries. All email activity is logged including mail sent from scripts. It does this by logging the current working directory from where the script was executed.

Using this knowledge you can easily track down a script of your own that is being exploited to send out spam or locate possibly malicious scripts that a spammer has placed onto your server.

Locate top scripts sending into Exim

In the steps below I’ll show how to locate the top scripts on your server sending mail. If any scripts look suspicious, you can check the Apache access logs to find how a spammer might be using your scripts send spam.

To follow the steps below you’ll need root access to your server, so you have access to the Exim mail log.

1. Login to your server via SSH as the root user.
2. Run the following command to pull the most used mailing script’s location from the Exim mail log:

   grep cwd /var/log/exim_mainlog | grep -v /var/spool | awk -F”cwd=” ‘{print $2}’ | awk ‘{print $1}’ | sort | uniq -c | sort -n

   Code breakdown:

   grep cwd /var/log/exim_mainlog	Use the grep command to locate mentions of cwd from the Exim mail log. This stands for current working directory.
   grep -v /var/spool	Use the grep with the -v flag which is an invert match, so we don’t show any lines that start with /var/spool as these are normal Exim deliveries not sent in from a script.
   awk -F”cwd=” ‘{print $2}’ | awk ‘{print $1}’	Use the awk command with the -Field seperator set to cwd=, then just print out the $2nd set of data, finally pipe that to theawk command again only printing out the$1st column so that we only get back the script path.
   sort | uniq -c | sort -n	Sort the script paths by their name, uniquely count them, then sort them again numerically from lowest to highest.

   You should get back something like this:

   15 /home/userna5/public_html/about-us
   25 /home/userna5/public_html
   7866 /home/userna5/public_html/data

   We can see /home/userna5/public_html/data by far has more deliveries coming in than any others.

3. Now we can run the following command to see what scripts are located in that directory:

   ls -lahtr /userna5/public_html/data

   In thise case we got back:

   drwxr-xr-x 17 userna5 userna5 4.0K Jan 20 10:25 ../
   -rw-r–r– 1 userna5 userna5 5.6K Jan 20 11:27 mailer.php
   drwxr-xr-x 2 userna5 userna5 4.0K Jan 20 11:27 ./

   So we can see there is a script called mailer.php in this directory

4. Knowing the mailer.php script was sending mail into Exim, we can now take a look at our Apache access log to see what IP addresses are accessing this script using the following command:

   grep “mailer.php” /home/userna5/access-logs/example.com | awk ‘{print $1}’ | sort -n | uniq -c | sort -n

   You should get back something similar to this:

   2 123.123.123.126
   2 123.123.123.125
   2 123.123.123.124
   7860 123.123.123.123

   We can see the IP address 123.123.123.123 was using our mailer script in a malicious nature.

5. If you find a malicious IP address sending a large volume of mail from a script, you’ll probably want to go ahead and block them at your server’s firewall so that they can’t try to connect again.

   This can be accomplished with the following command:

   apf -d 123.123.123.123 “Spamming from script in /home/userna5/public_html/data”

Hopefully, you’ve learned how to use your Exim mail log to see what scripts on your server are causing the most email activity. Also how to investigate if malicious activity is going on, and how to block it.

Originally posted on October 2, 2016 @ 1:40 pm