CVE-2013-3900: WinVerifyTrust Signature Validation Vulnerability

What is the result of opting into the stricter verification behavior?

Opting into the stricter verification behavior causes the WinVerifyTrust function to perform strict Windows Authenticode signature verification for PE files. After you opt in, PE files will be considered “unsigned” if Windows identifies content in them that does not conform to the Authenticode specification. This may impact some installers. If you are using an installer that is impacted, Microsoft recommends using an installer that only extracts content from validated portions of the signed file.

How can I enable the new signature verification behavior?

Customers who would like to enable the new Authenticode signature verification behavior can do so by setting a key in the system registry. When the key is set, Windows Authenticode signature verification will no longer recognize binaries with Authenticode signatures that contain extraneous information in the WIN_CERTIFICATE structure. Customers can choose to disable the functionality at any time by disabling this registry key. See Suggested Actions for instructions.

How can I disable the functionality?

Customers who have already enabled the stricter verification behavior, and have not experienced problems, can choose to leave the verification behavior enabled. Customers who are experiencing application compatibility problems with the new behavior, or customers who simply want to disable the new behavior, can disable the functionality by removing the EnableCertPaddingCheck registry key. See Suggested Actions for instructions.

If I do not want to enable this functionality, do I need to do anything?

No. The stricter verification behavior resides on the system but will be dormant functionality until enabled.

Does the new verification behavior affect already-installed software?

The new stricter verification behavior, when enabled, applies primarily to portable executable (PE) binaries that are signed with the Windows Authenticode signature format. Binaries that are not signed with this format or that do not use WinVerifyTrust to verify signatures are not affected by the new behavior. Binaries most likely to be affected are PE installer files distributed via the Internet that are customized at time of download. The most common scenario in which users could perceive an impact is during the downloading and installation of new applications. This is the case only if customers have chosen to enable the stricter verification behavior, after which users may observe warning messages when attempting to install new applications with signatures that fail validation.

Does the new verification behavior impact AppLocker policies?

For customers who have chosen to enable the stricter verification behavior, any AppLocker rule that depends on files being signed, or expects a specific publisher, may be impacted if the signature on a file does not meet the stricter Authenticode signature verification requirements.

Does the new verification behavior impact Windows Defender Application Control (WDAC)?

No. The new verification behavior does not impact WDAC.

Does the new verification behavior impact Software Restriction Policies?

For customers who have chosen to enable the stricter verification behavior, any Software Restriction Policy that depends on files being signed, or expects a specific publisher, may be impacted if the signature on a file does not meet the stricter Authenticode signature verification requirements.

The new stricter verification behavior deems my binary non-compliant. What are my options?

If a binary is deemed non-compliant with the stricter Authenticode signature verification behavior, this will not be a problem on systems that have not had the new verification behavior enabled because Microsoft is not enforcing the stricter behavior by default. However, to correct problems with a binary failing validation on systems where the new verification behavior has been enabled, that binary will need to be re-signed with strict adherence to the Windows Authenticode Signature format and specifically not include extraneous information in the WIN_CERTIFICATE structure.

Is there any possibility of a signature being recognized as non-compliant with the stricter verification process if I sign using non-Microsoft-provided signing tools?

Yes. For customers opting to enable the stricter verification behavior, signing binaries with non-Microsoft-provided signing tools runs the risk of signatures being recognized as non-compliant with the stricter verification behavior. Using Microsoft products, or signature tools Microsoft provides, such as signtool.exe, helps to ensure that signatures are recognized as compliant.

What is Windows Authenticode?

Windows Authenticode is a digital signature format that is used to determine the origin and integrity of software binaries. Authenticode uses Public-Key Cryptography Standards (PKCS) #7 signed data and X.509 certificates to bind an Authenticode-signed binary to the identity of a software publisher. The term “Authenticode signature” refers to a digital signature format that is generated and verified using the WinVerifyTrust function.

What is Windows Authenticode signature verification?

Windows Authenticode signature verification consists of two primary activities: signature checking on specified objects and trust verification. These activities are carried out by the WinVerifyTrust function, which executes a signature check then passes the inquiry to a trust provider that supports the action identifier, if one exists. For more technical information regarding the WinVerifyTrust function, see WinVerifyTrust function. For an introduction to Authenticode, see Introduction to Code Signing.

Suggested Actions

Review Microsoft Root Certificate Program Technical Requirements

Customers who are interested in learning more about the topic covered in this advisory should review Windows Root Certificate Program - Technical Requirements.

Modify Binary Signing Processes

After reviewing the technical details underlying the change in Authenticode signature verification behavior, Microsoft recommends that customers ensure that their Authenticode signatures do not contain extraneous information in the WIN_CERTIFICATE structure. Microsoft also recommends that executables authors consider conforming their Authenticode-signed binaries to the new verification standard. Authors who have modified their binary signing processes and would like to enable the new behavior may do so on an opt-in basis. Windows Root Certificate Program - Technical Requirements for guidance.

Test the Improvement to Authenticode Signature Verification

Microsoft recommends that customers test how this change to Authenticode signature verification behaves in their environment before fully implementing it. To enable the Authenticode signature verification improvements, modify the registry to add the EnableCertPaddingCheck value as detailed below.

Warning Performing these steps to enable the functionality changes will cause non-conforming binaries to appear unsigned and, therefore, render them untrusted.

Note If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

To enable the functionality perform the following steps:

For 32-bit versions of Microsoft Windows

Paste the following text in a text editor such as Notepad. Then, save the file by using the .reg file name extension (for example, enableAuthenticodeVerification.reg).

Windows Registry Editor Version 5.00  
[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]   
"EnableCertPaddingCheck"="1"  

You can apply this .reg file to individual systems by double-clicking it.

Note You must restart the system for your changes to take effect.

For 64-bit versions of Microsoft Windows

Paste the following text in a text editor such as Notepad. Then, save the file by using the .reg file name extension (for example, enableAuthenticodeVerification64.reg).

Windows Registry Editor Version 5.00  
[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]   
"EnableCertPaddingCheck"="1"

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

You can apply this .reg file to individual systems by double-clicking it.

Note You must restart the system for your changes to take effect.

Impact of enabling the functionality change: Non-conforming binaries will appear unsigned and, therefore, be rendered untrusted.

How to disable the functionality. Perform the following to delete the registry value previously added.

For 32-bit versions of Microsoft Windows, paste the following text in a text editor such as Notepad. Then, save the file by using the .reg file name extension (for example, disableAuthenticodeVerification.reg).

Windows Registry Editor Version 5.00  
[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]   
"EnableCertPaddingCheck"=-

You can apply this .reg file to individual systems by double-clicking it.

Note You must restart the system for your changes to take effect.

For 64-bit versions of Microsoft Windows, paste the following text in a text editor such as Notepad. Then, save the file by using the .reg file name extension (for example, disableAuthenticodeVerification64.reg).

Windows Registry Editor Version 5.00  
[HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Wintrust\Config]   
"EnableCertPaddingCheck"=-

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

You can apply this .reg file to individual systems by double-clicking it.

Note You must restart the system for your changes to take effect.