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Windows supports the digitally signing of EXEs and other application files so that you can verify the provenance of software before it executes on your system. This is an important element in the defense against malware. When a software publisher like Adobe signs their application they use the private key associated with a certificate they’ve obtained from one of the major certification authorities like Verisign.
Later, when you attempt to run a program, Windows can check the file’s signature and verify that it was signed by Adobe and that its bits haven’t been tampered with such as by the insertion of malicious code.
Windows doesn’t enforce digital signatures or limit which publisher’s programs can execute by default, but you can enable that with AppLocker. As powerful as AppLocker potentially is, it is also complicated to set up, except for environments with a very limited and standardized set of applications. You must create rules for at least every publisher whose code runs on your system.
The good news, however, is that AppLocker can also be activated in audit mode. And you can quickly set up a base set of allow rules by having AppLocker scan a sample system. The idea with running AppLocker in audit mode is that you then monitor the AppLocker event log for warnings about programs that failed to match any of the allow rules. This means the program has an invalid signature, was signed by a publisher you don’t trust or isn’t signed at all. The events you look for are 8003, 8006, 8021 and 8024 and these events are in the logs under AppLocker as shown here:
If you are going to use AppLocker in audit mode for detecting untrusted software remember that Windows logs these events on each local system. So be sure you are using a SIEM with an efficient agent, like EventTracker, to collect these events or use Windows Event Forwarding.
Better yet, if you have EventTracker, don’t bother with AppLocker – use EventTracker’s automatic Digital Forensics Incident and Incident Response feature for unknown processes. EventTracker watches each process (and soon each DLL) that your endpoints load and checks the EXE’s hash against your environment’s local whitelist (which EventTracker can automatically build). If not found there, EventTracker checks it against the National Software Reference Library. If the EXE still isn’t found to be legit, EventTracker posts it to the dashboard for you to review. EventTracker automatically provides publisher information if the file is signed, and other forensics such as the endpoint, user and parent process. With one click you can check the process against anti-malware sites such as VirusTotal. EventTracker goes way beyond AppLocker in its ability to detect suspicious software and giving the tools and information to quickly determine if the program is a risk or not, including the use of digital signatures.
There are some other issues to be aware of, though, with digitally signed applications and certificates. Certificates are part of a very complicated technology called Public Key Infrastructure (PKI). PKI has so many components and ties together so many different parties there is unfortunately a lot of room for error. Here’s a brief list of what has gone wrong in the past year or so with signed applications and the PKI that signatures depend on:
So, certificates and code signing are far from perfect — show me any security control that is. I really encourage you to try out AppLocker in audit mode and monitor the warnings it produces. You won’t break any user experience, the performance impact is hardly measurable and if you are monitoring those warnings you might just detect some malware the first time it executes instead of the 6 months or so that it takes on average.
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