CVE-2013-3900 is a critical vulnerability affecting Microsoft Windows systems. This flaw resides in how the WinVerifyTrust function validates Authenticode digital signatures on executable files. Authenticode is Microsoft’s code-signing technology designed to confirm the origin and integrity of applications distributed over the internet.

This vulnerability, first addressed in Microsoft Security Bulletin MS13-098, allows attackers to tamper with a signed executable (PE file) by injecting malicious code without invalidating the original digital signature. It poses a severe risk, as it enables remote code execution (RCE), potentially granting attackers unauthorized control over the target machine.

How the Vulnerability Works

The root cause of this vulnerability lies in how the WinVerifyTrust function verifies digital signatures. Normally, executables with valid signatures are marked as trusted, ensuring that they have not been altered since signing. However, CVE-2013-3900 allows an attacker to modify the contents of a digitally signed file, adding malicious payloads without breaking the digital signature’s validity.

PE files can be repackaged with malicious content post-signing.

When a user opens such a tampered executable, it still appears to be validly signed, deceiving both users and security systems.

Attackers can leverage this flaw to bypass security defenses and trigger unauthorized actions on the user’s machine.

Impact of CVE-2013-3900

This vulnerability significantly compromises the trust model underlying Microsoft’s signature validation process. Exploitation scenarios include:

Remote Code Execution (RCE): An attacker could convince users to execute seemingly legitimate signed software, gaining complete control over the target system.

Privilege Escalation: If executed on privileged accounts or critical systems, the attacker could extend control beyond user-level access.

Supply Chain Attacks: Hackers could inject malware into legitimate software packages without breaking their signatures, which makes the vulnerability particularly useful in supply chain compromises.

Vulnerable systems include:

Windows 7, 8, 8.1, and RT

Windows Server versions 2003, 2008, 2012, and their R2 variants.

Testing Compatibility

Before deploying this change system-wide, it’s essential to test it on a few machines. Some legitimately signed but non-compliant software might stop working once the padding check is enforced. Thus, a phased rollout and thorough compatibility testing are recommended.

4. Best Practices to Avoid Exploitation
5. Apply Security Patches: Ensure that all systems have installed the MS13-098 update.
6. Monitor Software Integrity: Use tools that validate software signatures beyond basic WinVerifyTrust checks.
7. Educate Users: Train employees to avoid downloading and executing software from unverified sources, even if it appears to be signed.
8. Implement Endpoint Protection: Use endpoint protection systems to detect unusual behavior triggered by rogue executables.
9. Enable Strict Validation Policies: Enforcing the registry changes improves security, but administrators must monitor for compatibility issues.

Risk vs. Compatibility Considerations

Enabling the certificate padding check introduces stricter validation, which may block older or improperly signed software. Microsoft’s decision to leave the stricter checks disabled by default reflects a trade-off between security and compatibility. Organizations must decide based on their risk tolerance whether to enforce the check universally or selectively for critical systems.

Mitigation and Workarounds

Microsoft’s Patch (MS13-098)

Microsoft released MS13-098 to address this flaw, but the patch does not enforce strict validation by default. The patch ensures compatibility with software that uses non-compliant signatures, requiring administrators to enable additional checks for full protection.

Enabling Certificate Padding Check via Registry

For stricter validation, administrators can enforce the certificate padding check using Windows Registry settings. This action blocks non-conforming binaries from appearing as signed.

How to Enable Certificate Padding Check (Registry Change)

Here’s how to strengthen your system’s protection by ensuring modified files appear unsigned, even if they carry a digital signature.

1. Open Registry Editor:
   • Press Windows + R, type regedit, and press Enter.
2. Add the following registry keys to enable the certificate padding check

[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]
"EnableCertPaddingCheck"="1"

[HKEY_LOCAL_MACHINE\Software\Wow6432Node\Microsoft\Cryptography\Wintrust\Config]
"EnableCertPaddingCheck"="1"

1. Restart the System:
   • A system reboot is required to apply the changes fully.

Explanation:

1. Value Type: According to Microsoft’s recommendations, the "EnableCertPaddingCheck" must be set as a string (REG_SZ) type with the value "1". This instructs Windows to enable the signature padding check for certificates, mitigating the vulnerability.
2. Registry Paths: Two paths are specified to ensure compatibility for both 32-bit and 64-bit environments on Windows:
   • One for standard 64-bit registry space.
   • Another under Wow6432Node for 32-bit applications running on 64-bit systems.

Why REG_SZ Matters:

This configuration was explicitly outlined in discussions around CVE-2013-3900, where some users initially tried DWORD but faced issues. The correct value type ensures the security mechanism activates properly​

After applying these changes, make sure to restart the system to ensure the settings take effect. If further testing or verification is required, tools like Intune can also deploy and monitor these registry changes across multiple devices​

These changes ensure that executables modified post-signing are treated as unsigned, improving the security of the system.

PowerShell and REG File Method

If manual entry is challenging, use a .reg file or PowerShell command:

• REG File: Copy the above registry content into a .reg file and double-click to execute.
• PowerShell Command:

[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]
"EnableCertPaddingCheck"="1"
[HKEY_LOCAL_MACHINE\Software\Wow6432Node\Microsoft\Cryptography\Wintrust\Config]
"EnableCertPaddingCheck"="1"

Once you done executing you will see something like this

If the REG_SZ type is created by default via the PowerShell command, it is not an issue, as REG_SZ is the correct type required for EnableCertPaddingCheck.

Conclusion

CVE-2013-3900 is a stark reminder of how even fundamental security mechanisms like digital signatures can be compromised if not properly validated. Although Microsoft has released a patch through MS13-098, the onus is on administrators to enforce stricter validation by modifying registry settings. Implementing the padding check ensures better protection but may impact some software compatibility. Balancing security with functionality is critical, and organizations should carefully evaluate their security posture to determine the most suitable mitigation strategy.

References

1. Microsoft Documentation on CVE-2013-3900: Microsoft Learn
2. Tenable Advisory on MS13-098: Tenable

These links provide additional technical guidance and help in understanding the vulnerability better.