Flame and Certificate Revocation

Microsoft has published patches that insert the CAs directly associated with the Terminal Services Licensing PKI into the “Untrusted” certificate store, this has the same effect as revoking the certificates for those that:

  1. Apply the patch
  2. Use Microsoft’s chain validation engine

But what about those that do not apply the patch? Or those that simply import the roots from the Microsoft store to use in another engine (like NSS or OpenSSLs, one example of such an application would be Adobe X)?

Well today those people will still trust certificates issued by those certificates, they potentially would not though if Microsoft published revocation information for those CAs.

As it stands most of the certificates in question do contain revocation pointers (a CRLdp extension in this case) but the URLs they point to are invalid, for example:

  1. “Microsoft Enforced Licensing Intermediate PCA” – No revocation information
  2. “Microsoft Enforced Licensing Registration Authority CA” – http://crl.microsoft.com/pki/crl/products/MicEnfLicPCA_12-10-09.crl
  3. “Microsoft LSRA PA” –  http://crl.microsoft.com/pki/crl/products/MicEnfLicRegAutCA_2009-12-10.crl
  4. “Terminal Services LS” – http://tk5paxprdsrv09.partners.extranet.microsoft.com/CertEnroll/Microsoft%20LSRA%20PA.crl

This may not be beneficial for the attack vector used in Flame (I do not know if it even does revocation checking – though it very probably does) but it would certainly help other cases.

Today the invalid URLs in these certificates would result in a “Unknown or Offline” response which would likely be ignored by applications due to Microsoft’s “soft-fail revocation checking” defaults.

Even with those defaults though if they published these CRLs clients would get some benefit, it would:

  1. Reduce risk until the patch could get applied.
  2. Help others who mistakenly trusted the Microsoft roots when they “imported” them from the “Trusted Root Store”.

It’s my hope Microsoft decides to publish these CRLs for relying party applications to rely on.

[3:22 PM 6/19/2012] FYI as of 6/19 the MicEnfLicPCA_12-10-09.crl is published (good), it appears to have been created June 14th several days after this post. The other two are still invalid, it’s likely they decided against publishing them since a well behaved client would fail with just one of the CAs being bad. It’s not a unreasonable decision, I am guessing the URL choices for the CRLs made it difficult to do all of the CRLs.


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