Sguil: adding support for ModSecurity alerts, continued

After the successful test with the Perl script to add ModSecurity alerts to Sguil, I have been working on a more robust implementation, also in Perl. Let me first describe the basic setup. The setup works with two scripts. The first places links to event files into a special queue directory. The second reads the links from that directory, parses them and sends the alerts among these events to Sguil. After that, the links are removed.

In ModSecurity there is a logging method called concurrent. What it does is that it creates a directory structure based on date and time. For each event a file is created in the directory structure, with the details of the event. It also writes a logfile called index, that adds one line for every event, with location of the file in the date/time directory layout.

ModSecurity also supports using a external script to take care of this indexing task, which is what the script modsec-auditlog-collector in the ModSecurity source tarball does. I have modified this Perl script to create a link to the actual event file in a special queue directory. The links all have a name like: modsec.log.-timestamp-, like Snort uses with unified logging. I have chosen to use links because I think it might be faster than copying, but I have yet to benchmark that.

The second script takes care of processing the links and communicating with Sguil. It basically has the following workflow. It gets a list of all the files in the queue. Each file in the queue is parsed, then it is checked whether it is an alert. If so, the data is preprocessed to make sure it is what Sguil expects. Next it is send to Sguil. Finally, all the files are removed.

ModSecurity has a different rule syntax than Snort and sees a different type of data. Snort sees packets, and thus knows about TCP checksums, sequence numbers, etc. ModSecurity works on a higher level, so it has no access to that data. One thing that ModSecurity alerts do include is the action taken: blocked or redirected and with what HTTP code. If available the rule msg is used for the Event Message, otherwise the ModSecurity message with (a part of) the rule that matched. Below is a screenshot that shows both, and also the HTTP code.

Sguil and Mod_Security msg view

The current implementation acts as a drop-in replacement of Barnyard. This way no changes to Sguil are necessary. Currently this means that Barnyard cannot run on the same sensor, although multiple instances of sensor_agent on a host could fix that. Sguil author Bamm Visscher suggested I can try to connect to the Sguil server directly instead of via the sensor_agent to fix this. I will probably have a look at that later.

Another issue is the way the alert data is presented in the Sguil interface. Currently I attach the entire alert file to the alert that is being send to Sguil, as the packet payload. However the Sguil packet payload view is not very usable for this kind of data, as can be seen on the screenshot below.

Sguil and Mod_Security payload view

So here is room for much improvement. Ideally Sguil would be able to show the uri, the post payload if applicable, useragent, etc. I’ve never worked with TCL/TK, but I did use Perl/TK in the past, so that might help me a little if I decide to give it a shot.

I expect a public beta version to be ready some time next week, although I have not yet decided how to release it. I could create a project on SourceForge or just have a simple webpage. I just haven’t decided yet. Stay tuned…

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