Microsoft Security Advisory 3123479

Deprecation of SHA-1 Hashing Algorithm for Microsoft Root Certificate Program

Published: January 12, 2016

Version: 1.0

Microsoft has released a SHA-1 code sign deprecation change effective January 1, 2016, focused on client activity that can only occur when a customer downloads files from the Internet. This change is specific to a new default setting for Windows and customers can override or augment the default settings in their environment.

For customers running either Internet Explorer or Microsoft Edge who download a SHA-1 signed file from the Internet that is timestamped and released on January 1, 2016, or later, SmartScreen will mark the file as not trusted. This status does not prevent customers from downloading the file or running these browsers on their computers. But customers are warned of the not trusted status of the file.

This change only affects Mark-of-the-Web (MOTW) files downloaded from the Internet. Files timestamped before January 1, 2016, will continue to be trusted. Drivers with signatures verified by Code Integrity are not affected by this change. To conform to the latest requirements for driver signing, see the Windows Hardware Certification blog.

Issue References

For more information about this issue, see the following references:



General Information

Windows Enforcement of Authenticode Code Signing and Timestamping

Technical Requirements

Protecting Against Weak Cryptographic Algorithms

This advisory applies to the following operating systems:

Windows 7

Windows 7 for 32-bit Systems Service Pack 1

Windows 7 for x64-based Systems Service Pack 1

Windows Server 2008 R2

Windows Server 2008 R2 for x64-based Systems Service Pack 1

Windows Server 2008 R2 for Itanium-based Systems Service Pack 1

Windows 8

Windows 8 for 32-bit Systems

Windows 8 for x64-based Systems

Windows Server 2012

Windows Server 2012

Windows 8.1

Windows 8.1 for 32-bit Systems

Windows 8.1 for x64-based Systems

Windows Server 2012 R2

Windows Server 2012 R2

Windows 10

Windows 10 for 32-bit Systems[1]

Windows 10 for x64-based Systems[1]

Windows 10 Version 1511 for 32-bit Systems[1]

Windows 10 Version 1511 for x64-based Systems[1]

Server Core installation option

Windows Server 2008 R2 for x64-based Systems (Server Core installation)

Windows Server 2012 (Server Core installation)

Windows Server 2012 R2 (Server Core installation)

[1]The Windows 10 update is cumulative. In addition to containing non-security updates, it also contains all of the security fixes for all of the Windows 10-affected vulnerabilities shipping with given month’s security release. The update is available via the Windows Update Catalog. See Microsoft Knowledge Base Article 3097617 for more information and download links.

What is the scope of the advisory? 
This advisory aims to assist customers in assessing the risk of certain applications that use X.509 digital certificates that are signed using the SHA-1 hashing algorithm and to recommend that administrators and certificate authorities begin using SHA-2 in place of SHA-1 as an algorithm for signing digital certificates.

Is this a security vulnerability that requires Microsoft to issue a security update? 
No. A signing mechanism alternative to SHA-1 has been available for some time, and the use of SHA-1 as a hashing algorithm for signing purposes has been discouraged and is no longer a best practice. Microsoft will however evaluate any opportunities to strengthen technologies to detect fraudulent certificates. Although this is not a vulnerability in a Microsoft product, Microsoft is issuing this advisory to help clarify the actual risk involved to customers.

What causes this threat?  
The root cause of the problem is a known weakness of the SHA-1 hashing algorithm that exposes it to collision attacks. Such attacks could allow an attacker to generate additional certificates that have the same digital signature as an original. These issues are well understood and the use of SHA-1 certificates for specific purposes that require resistance against these attacks has been discouraged. At Microsoft, the Security Development Lifecycle has required Microsoft to no longer use the SHA-1 hashing algorithm as a default in Microsoft software. For more information about SHA-1 collision weakness, see The SHAppening: freestart collisions for SHA-1.

What is a digital certificate? 
In public key cryptography, one of the keys, known as the private key, must be kept secret. The other key, known as the public key, is intended to be shared with the world. However, there must be a way for the owner of the key to tell the world who the key belongs to. Digital certificates provide a way to do this. A digital certificate is an electronic credential used to certify the online identities of individuals, organizations, and computers. Digital certificates contain a public key packaged together with information about it - who owns it, what it can be used for, when it expires, and so forth. For more information, see Understanding Public Key Cryptography and Digital Certificates.

What is the purpose of a digital certificate?  
Digital certificates are used primarily to verify the identity of a person or device, authenticate a service, or encrypt files. Normally, there is no need to think about certificates at all, aside from the occasional message stating that a certificate is expired or invalid. In such cases, one should follow the instructions provided in the message.

What is a certification authority (CA)?  
Certification authorities are the organizations that issue certificates. They establish and verify the authenticity of public keys that belong to people or other certification authorities, and they verify the identity of a person or organization that asks for a certificate.

  • Review Microsoft Root Certificate Program Policy Changes

    Customers who are interested in learning more about the topic covered in this advisory should review Windows Enforcement of Authenticode Code Signing and Timestamping.

  • Update from SHA-1 to SHA-2

    Certificate authorities should no longer sign newly generated certificates using the SHA-1 hashing algorithm. Customers should ensure that their certificate authorities are using the SHA-2 hashing algorithm to obtain SHA-2 certificates from their certificate authorities. To sign code with SHA-2 certificates, see the guidance on this topic at Windows Enforcement of Authenticode Code Signing and Timestamping.

    Impact of action: Older hardware-based solutions may require upgrading to support these newer technologies.

  • Keep Windows Updated

    All Windows users should apply the latest Microsoft security updates to help make sure that their computers are as protected as possible. If you are not sure whether your software is up to date, visit Windows Update, scan your computer for available updates, and install any high-priority updates that are offered to you. If you have Automatic Updates enabled, the updates are delivered to you when they are released, but you have to make sure you install them.




The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.


  • V1.0 (January 12, 2016): Advisory published.

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