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Microsoft Security Advisory (961509)

Research proves feasibility of collision attacks against MD5


Microsoft is aware that research was published at a security conference proving a successful attack against X.509 digital certificates signed using the MD5 hashing algorithm. This attack method could allow an attacker to generate additional digital certificates with different content that have the same digital signature as an original certificate. The MD5 algorithm had previously shown a vulnerability, but a practical attack had not yet been demonstrated.

This new disclosure does not increase risk to customers significantly, as the researchers have not published the cryptographic background to the attack, and the attack is not repeatable without this information. Microsoft is not aware of any active attacks using this issue and is actively working with certificate authorities to ensure they are aware of this new research and is encouraging them to migrate to the newer SHA-1 signing algorithm.

While this issue is not a vulnerability in a Microsoft product, Microsoft is actively monitoring the situation and has worked with affected Certificate Authorities to keep customers informed and to provide customer guidance as necessary.

Mitigating Factors:

  • Microsoft is not aware of specific attacks against MD5, so previously issued certificates that were signed using MD5 are not affected and do not need to be revoked. This issue only affects certificates being signed using MD5 after the publication of the attack method.
  • Most public Certificate Authority roots no longer use MD5 to sign certificates, but have upgraded to the more secure SHA-1 algorithm. Customers should contact their issuing Certificate Authority for guidance.
  • When visited, Web sites that use Extended Validation (EV) certificates show a green address bar in most modern browsers. These certificates are always signed using SHA-1 and as such are not affected by this newly reported research.

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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.


  • December 30, 2008: Advisory published