With the exception of Role-Based Access Control (RBAC), File Integrity Monitoring (FIM) is the only PCI requirement that achieves security in its purest form; prevention of, or alerts on, deviation from a known-good baseline.
Firewalls/Routers (DSS Req. 1.x) is almost there, configuration standards is even closer (DSS Req. 2.x), anti-virus (DSS Req. 5.x) is basically pointless, and logging (DSS Req. 10.x) could not be further off the mark. But FIM, assuming you have interpreted the requirements correctly, has real benefit when combined with the other requirements done well (especially configurations).
Unfortunately, even to this day, everyone associates Tripwire ® with the FIM requirement. If you can believe it, their NAME was included in version 1.0 of the PCI DSS! Of course, Tripwire ® licensing fees went through the roof as a result, and I’ve pretty much hated them for it ever since. Now they’re jumping on the Security Incident and Event Management (SIEM) bandwagon and doing as bad a job of it as everyone else.
PCI DSS v1.0 – “10.5.5 Use file integrity monitoring/change detection software (such a Tripwire ®) on logs to ensure that existing log data cannot be changed without generating alerts (although new data being added should not cause an alert).”
Couldn’t even spell “as” correctly, but I digress.
As in all DSS requirements, the first question you must ask yourself is; “What is the intent of…?” In this case, the intent of FIM is to ensure all of your critical files (both operating system and application) do not change without authorisation (i.e. outside of a known change control scenario). Basically it’s seen by the SSC as a back-up for anti-virus (malware being the primary cause of unauthorised file changes), but in my opinion, the correct implementation of FIM, configuration standards, and baselined logging more of less negates AV altogether (seeAnnual Validation is Dead, it’s Time for Continuous Compliance, Continuous Compliance Validation: Why The PCI DSS Will Always Fall Short, and PCI – Going Beyond the Standard: Part 10, Anti-Virus for a little more background).
PCI DSS Requirement 11.5 states; “11.5 Deploy a change-detection mechanism (for example, file-integrity monitoring tools) to alert personnel to unauthorised modification of critical system files, configuration files, or content files; and configure the software to perform critical file comparisons at least weekly.” so it should be clear already how to go above and beyond; perform the checks more frequently that weekly. For a start, weekly is ridiculous, a lot can happen in a week, but the requirements’ very limited benefit is compounded by the fact that there is no guidance in WHAT changes you should be looking for.
FIMs can usually detect everything from file existence, size, permissions, hash values and so on, but these are only making checks against themselves from a previous ‘run’. Therefore one of the best ways to go WAY above PCI minimums is to compare the files to central database of known good configs directly from the operating system vendor themselves. Microsoft has a database of the latest and greatest system files (DLLS, EXEs etc.) against which you can run comparisons, and it should be relatively simple to add the baselines from each application you install on top.
Now let’s get REALLY crazy; What if you could then compare what files SHOULD be there as a result of a comprehensive configuration standard/hardening guide, and ensure that everything is as it should be? Against CIS Security Benchmarks for example? Some FIMs (or similar agents) can also check Windows registry and GPO settings, so you can not only make sure everything is configured correctly per your approved standard(s), you can also automatically report against a significant number of other validation requirements (password complexity, access groups, log settings, time synch settings and so on).
And of course, the best way to blow PCI minimums out of the water is to compare a system against baselines stored centrally against each asset. These are system x’s available services, listening ports, permitted established connections and so on. Now you not only have configuration management, you have both policy and compliance validation built in automatically. Not once a year, but all day every day.
OK, I went WAY too far there, but hopefully, you get the point. FIM is not just something you throw on a system because PCI demands it, you do it because it’s integral to how you do real security properly.
Yes, Tripwire ® can do more than PCI asks for, but now you have just another management station to configure, maintain and monitor. FIM done well must integrate with all your other systems to have the necessary context, and FIM is only relevant if you have configured your systems correctly in the first place.
Finally, FIM should NEVER be seen as a stand-alone, end-point product, it can and should be a lot more than that.