Using AIDE for file integrity monitoring (FIM) on CentOS

PCI-DSS 3.1 section 10.5.5 has the following requirement:

Use file-integrity monitoring or change-detection software on logs to ensure that existing log data cannot be changed without generating alerts (although new data being added should not cause an alert).

For large solutions, I would suggest using a well known tool such as Tripwire Enterprise. However many small to mid size companies that have a small footprint within their card holder data environment (CDE), may not be able to afford this. So what can companies use to meet this requirement? Implement AIDE (Advanced Intrusion Detection Environment).

Taken from the projects website, AIDE creates a database from the regular expression rules that it finds from the config file(s). Once this database is initialized it can be used to verify the integrity of the files.

AIDE is a very simple (yet powerful) program that runs from cron checking your files (typically once a night), and it will scan your system looking for any changes in the directories its monitoring. There are a number of different ways to use this program, but I’ll outline one that I like to use.

My requirements:
1. I want the reports to run nightly.
2. All change reports are emailed to me so I can archive them for a year offsite.
3. Have the database automatically commit the additions, deletions, and changes to baseline each time its ran.

In the event my system was compromised, I want to ensure that the malicious user was not able to modify, or delete my previous reports. Therefore, I choose not to store them on the machine. Its true that once the malicious user gained access to my system, they could change my AIDE config on me, but at least my previous reports will be intact which should help me when determining what malicious changes this user made to my server. Please note that I am making an assumption here that you are already backing up your system nightly, which would include your AIDE database! If you do not currently have a backup strategy in place, get one. Tools such as AIDE helps identify what files a malicious user may have changed, but if they completely crippled the system, you will need to restore from backups.

Setting up AIDE is fairly straight forward. It exists in most of package repositories out there including most variants of Linux and BSD.

On Linux based systems, you can install it by:

[[email protected] ~]# yum install aide

Once you have AIDE installed, the default configuration from the upstream provider should give you a reasonable default aide.conf. But what if you wanted to add your website documentroot to this so you can keep track of what files are changing on your website? Well, we simple add the directory to the aide.conf by including:

[[email protected] ~]# vim /etc/aide.conf
...
/var/www/vhosts/domain.com
...

Now AIDE will be keeping track of our website. But adding your site may lead to very noisy reports because most websites implement caching. So this now becomes a balancing act to exclude directories that change often, yet retain enough of your sites critical content. We could just leave the entire directory in AIDE, but I know I personally don’t want to read a change report that contains 1,000 changes every day. So in the case of this wordpress site, I exclude the cache directory by appending the following to my custom configuration:

[[email protected] ~]# vim /etc/aide.conf
...
/var/www/vhosts/domain.com
!/var/www/vhosts/domain.com/web/wp-content/cache
...

The “!” means NOT to monitor that specific directory. You will need to run AIDE a few times and fine tune the configuration before you get a report that is useful for your specific needs.

On CentOS, I had to change the following setting in /etc/aide.conf for the initialization to work:

[[email protected] ~]# vim /etc/aide.conf
...
# Whether to gzip the output to database
gzip_dbout=no
...

Once you have your configuration tuned for your specific purposes, you first my initialize the database to create a baseline before you can start getting change reports. You do this by:

[[email protected] ~]# aide --init
[[email protected] ~]# mv -f /var/lib/aide/aide.db.new /var/lib/aide/aide.db

Now, try making a basic change to /etc/hosts, then run a check on AIDE to see if it detects the change:

[[email protected] ~]# aide --check

If you are like me and would prefer not to have to log into 10 servers a day to run and view the reports, you can configure cron to run the report, and email you the results daily, while committing the changes to baseline. If you choose to go this route, it is critical that you review your change reports as they come in because we are essentially committing every change to the baseline. Here is how I configure cron:

[[email protected] ~]# crontab -e
# Perform daily change report
0 3 * * * /usr/sbin/aide --update | mail -s "AIDE Audit Report : web01.example.com" [email protected]

# Initialize the AIDE database once a day:
30 3 * * *  nice -19 /usr/sbin/aide --init;mv -f /var/lib/aide/aide.db.new /var/lib/aide/aide.db

Posted below is an example report that AIDE would send me via email daily:

AIDE found differences between database and filesystem!!
Start timestamp: 2012-09-13 01:24:05

Summary:
Total number of files: 57620
Added files: 1
Removed files: 1
Changed files: 1

---------------------------------------------------
Added files:
---------------------------------------------------

added: /var/spool/cron/root

---------------------------------------------------
Removed files:
---------------------------------------------------

removed: /etc/.aide.conf.swp

---------------------------------------------------
Changed files:
---------------------------------------------------

changed: /etc/aide.conf

--------------------------------------------------
Detailed information about changes:
---------------------------------------------------

File: /etc/aide.conf
Size : 2381 , 2390
Mtime : 2012-09-13 01:24:05 , 2012-09-13 01:24:05
Ctime : 2012-09-13 01:24:05 , 2012-09-13 01:24:05
MD5 : b+qbBDYEPesd+NCR1VRQHQ== , rG5pNPghdweedpU/c0ieHw==
RMD160 : T081ixhqik4efC3dfeCOBDCKpP4= ,
qe8MV0eteklAKmlZ5LTubaOUNKo=
SHA256 : g4jstEtfU8BNu+43jkrxJc9Cpr2SABZj ,
a65iaV54XR4vu8/zbA4Tdfe2U+W5uPNY

So this reports tells me that root’s crontab was added, a swap file for aide.conf was removed, and I updated the /etc/aide.conf recently.

Please remember that utilizing a tool to provide file integrity monitoring is only one part of a defense in depth strategy. There is no silver bullet for system security, but every layer you add will increase your security footprint which helps you with taking a proactive approach to security.