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How can Windows digital signature check be defeated?

A security researcher discovered that editing two registry keys can alter a Windows digital signature check. Matt Pascucci explains what that means for digital signatures.

A DerbyCon 7.0 keynote revealed a simple way to defeat a Windows digital signature check by editing just two registry...

keys. How important is Windows digital signature protection, and how difficult would it be for attackers to make these registry key changes?

Recently, it was determined by a SpecterOps researcher, Matt Graeber, that there is a way to bypass a Windows digital signature check by editing two specific registry keys. This is an important discovery because Windows uses digital signature protection to validate the authenticity of binary files as a security measure.

Digital signature protection is used by Windows and others to determine if a file was tampered with during the time in which it was sent to the receiving party. Being able to validate the integrity of a received file and that it's actually from the party that signed it is important since digital signatures work on trust -- when a system can work around this feature, it opens up doors to malicious activity.

It's also important to state that digital signatures don't secure the file, but give it a level of trust based off of the private key it was signed with; therefore, if that specific key was stolen or used maliciously, then the system would approve the digital signature check.

Many Windows security features and security products rely on the trust and guarantees that a digital signature check brings with it. In the case of the CCleaner malware last month, it spread due to having been signed by a legitimate certificate, which led to the code being trusted by the OS. In his research report, Graeber wrote, "Subverting the trust architecture of Windows, in many cases, is also likely to subvert the efficacy of security products."

The attack is focused on two registry keys that enable you to impersonate files with any other valid signature when adjusted. However, this isn't done via injection of code into the system, but with the registry key modification, meaning the attacker can do this remotely if they have access to the registry. This also means that they must be admins on the system, which isn't incredibly hard to escalate if they aren't don't have permission.

Locking down the administrator rights to limit changes to these keys and implementing monitoring to determine if they were changed would be a way of reviewing if the registry keys were modified, even though this would require the logs of all the systems. It's also possible that a group policy could be made to limit access to these files in greater detail, but these are all reactive methods to this problem.

The issue once again comes down to trust, as this is one area that's put in place to protect you from impersonation. It also happens to be the most likely thing to be used for malicious purposes, especially malware, that would bypass the internal mechanisms to slip past application whitelisting, such as Microsoft's Windows Defender Device Guard.

There needs to be more procedures around digital signature protection to protect against malicious files entering your endpoint.

There needs to be more procedures around digital signature protection to protect against malicious files entering your endpoint, such as reputation services, sandboxes and next-generation malware protection that doesn't rely on signatures.

Is a digital signature check needed? Yes, but it's a layer in the protection against malware, and abusing the trust of these signatures enables them to be bypassed. In the end, we simply need to add more layers to our defense

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This was last published in December 2017

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