CVE-2013-3900 PoC — WinVerifyTrust Signature Validation Vulnerability

Source

https://github.com/pkblanks/Remediating-CVE-2013-3900-EnableCertPaddingCheck-

Associated Vulnerability

Title:WinVerifyTrust Signature Validation Vulnerability (CVE-2013-3900)
Description:Why is Microsoft republishing a CVE from 2013? We are republishing CVE-2013-3900 in the Security Update Guide to update the Security Updates table and to inform customers that the EnableCertPaddingCheck is available in all currently supported versions of Windows 10 and Windows 11. While the format is different from the original CVE published in 2013, except for clarifications about how to configure the EnableCertPaddingCheck registry value, the information herein remains unchanged from the original text published on December 10, 2013, Microsoft does not plan to enforce the stricter verification behavior as a default functionality on supported releases of Microsoft Windows. This behavior remains available as an opt-in feature via reg key setting, and is available on supported editions of Windows released since December 10, 2013. This includes all currently supported versions of Windows 10 and Windows 11. The supporting code for this reg key was incorporated at the time of release for Windows 10 and Windows 11, so no security update is required; however, the reg key must be set. See the Security Updates table for the list of affected software. Vulnerability Description A remote code execution vulnerability exists in the way that the WinVerifyTrust function handles Windows Authenticode signature verification for portable executable (PE) files. An anonymous attacker could exploit the vulnerability by modifying an existing signed executable file to leverage unverified portions of the file in such a way as to add malicious code to the file without invalidating the signature. An attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. If a user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights. Exploitation of this vulnerability requires that a user or application run or install a specially crafted, signed PE file. An attacker could modify an... See more at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2013-3900

Readme

 # **Remediation Report: CVE-2013-3900 on Windows Server 2019 (Azure VM)**

Remediating CVE-2013-3900 (EnableCertPaddingCheck)

During an authenticated vulnerability scan using Tenable/ Nessus, I identified CVE-2013-3900 on my Windows Server 2019 Datacenter instance hosted in Microsoft Azure. The scan utilized valid administrative credentials to perform in-depth checks, confirming the presence of this vulnerability at the OS level.

CVE-2013-3900 is a WinVerifyTrust signature validation vulnerability that can allow attackers to bypass digital signature verification, particularly through maliciously modified PE files.

### **Next Action**:
To mitigate this vulnerability, I plan to apply Microsoft’s recommended registry-level fix, which enforces stricter signature verification. 
I will also validate whether this mitigation can be applied safely in our environment without breaking application compatibility.

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<img width="300" alt="image" src="https://github.com/pkblanks/attachments/blob/main/tenable-logp.jpg">
<img width="300" alt="image" src="https://github.com/pkblanks/attachments/blob/main/Azure%20Image.jpg">


## Technology Utilized
- Tenable (enterprise vulnerability management platform)
- Azure Virtual Machines (Nessus scan engine + scan targets
- command prompt (CMD - remediation scripts)
---

## **Discovery**
During a credentialed/ authenticated vulnerability scan using Tenable/ Nessus, the internal scan engine flagged CVE-2013-3900 — a known high-severity vulnerability — on a Windows Server 2019 Datacenter instance hosted in Microsoft Azure.
Nessus classifies this vulnerability as High, due to its ability to bypass digital signature verification, enabling malicious payloads to appear as trusted code.
The initial internal scan results using Nessus can be seen below: 

<img width="750" alt="image" src="https://github.com/pkblanks/attachments/blob/main/1.CVE-2013-3900.jpg">


## **Remediation Steps**:
To mitigate this vulnerability, the following registry-level configuration was applied:

### Step 1: Open Command Prompt as Administrator
Search for cmd, right-click on Command Prompt, and select Run as administrator.

### Step 2: Apply Registry Changes by pasting the commands below in CMD (execute them step by step):

1. reg add "HKLM\Software\Microsoft\Cryptography\Wintrust\Config" /v EnableCertPaddingCheck /t REG_DWORD /d 1 /f

2. reg add "HKLM\Software\Wow6432Node\Microsoft\Cryptography\Wintrust\Config" /v EnableCertPaddingCheck /t REG_DWORD /d 1 /f

These registry keys enable strict certificate padding checks by setting EnableCertPaddingCheck to 1, thus preventing attackers from exploiting padding flaws in signed PE files.

### Step 3: Reboot System
A system reboot was performed to ensure the changes applied across all processes and services.

### Step 4: Perform final credentialed scan 
After a successful credentialed scan , we can now observe that the vulnerability has been eliminated as shown below:

<img width="750" alt="image" src="https://github.com/pkblanks/attachments/blob/main/2.CVE-2013_3900.jpg">


### ** Post-Remediation Validation**
Following remediation, a subsequent authenticated Nessus scan confirmed that CVE-2013-3900 has been fully resolved.

⚠️ **Risks of Leaving CVE-2013-3900 Unpatched**:
1. Bypass of Digital Signature Verification
2. Malicious code can be inserted into signed files without invalidating their signature.
3. Evasion of Detection Mechanisms
4. Malware can bypass AV, EDR, and AppLocker controls by appearing as trusted software.
5. Privilege Escalation & Remote Code Execution
6. Exploitation may lead to installation of backdoors, data exfiltration, and lateral movement.
7. Software Supply Chain Vulnerability
8. Attackers could tamper with signed installers or updates during distribution.
9. Compliance & Legal Exposure
10. Unpatched known vulnerabilities can result in violations of NIST, CMMC, HIPAA, PCI DSS, and other frameworks — increasing regulatory risk and liability in the event of a breach.

## **Summary**
**CVE-2013-3900 is a critical stealth attack vector that undermines the trust model of digitally signed software. Through prompt remediation and verification, this system is now hardened against this threat.**

**Regular credentialed scanning, combined with patch validation and configuration enforcement, remains a cornerstone of our Vulnerability Management Program.**

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[View report here](https://docs.google.com/document/d/1e-aR30eBCeWf-Ucg2GemXJjWkUjGz4AID4gm7CDEI6Y/edit?usp=sharing)

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