How to cause a DoS in Windows 8 explorer.exe

Monday, September 30, 2013

We have discovered by accident how to cause a Denial of Service (DoS) in Windows 8. It’s a little bug that is present in the last version of the operating system. Since we alerted Microsoft first and they didn’t consider it a real security problem that could be attacked we’re documenting it as an anecdote.

Explorer.exe is a very important service in Windows. It’s in charge of painting the desktop and gives the security tokens to the programs that are in the same environment. It’s of vital importance that it’s running in every moment, hence if the process dies for some reason, the operating system itself will recover it automatically.

Seemingly, in Windows 8, explorer.exe doesn’t handle correctly an exception that is thrown when dealing with digital certificates and it forces it to close and launch again. This problem also affects other programs that use the same internal API that processes ASN.1 structures. For example, any program that uses .NET and processes the “signedInfo” field of a signature.

These are steps to reproduce the problem:
  • Have a signed binary (DLL or EXE) at hand. Any binary is valid if it’s signed.
  • Fill the last section of the PKCS structure with zeroes or random values. For example 256 bytes of “00”.

A part of the signature filled with 00s

In this example we’ve overwritten part of the information regarding the countersignature as we can observe when opening the ASN.1 structure with a different program. We haven’t tested exactly which part causes the problem when being overwritten.

On the left, altered ASN.1 structure, on the right, unaltered structure.

If we overwrite other kind of information Windows will simply think that the binary isn’t signed and won’t show the “Digital signatures” tab in the properties dialog.

  • Using Explorer to access the “Digital signatures” tab will crash explorer.exe with an unhandled exception. Other programs like “Total commander” also crash in the attempt of showing the certificate. This bug is only present in Windows 8. The same proof of concept in Windows XP/7 only tricks the system to show the “Digital signatures” tab without any info to display. This isn’t normal either (the tab shouldn’t be visible) but at least it doesn’t kill the process.

Other programs that check the signature such as sigcheck or signtool are not affected.
In theory this can be related to the change of design. In Windows 7 and XP the email of the signer is shown in the “Digital signatures” information tab. In Windows 8 the hash is being shown. We suppose that they became aware that very few signers include the email in the signature, and this field was usually blank.

On the left, properties of a signed file in Windows 7. On the right, in Windows 8.

A quick analysis results in our hypothesis that it’s difficult to take advantage of the bug to run arbitrary code. MSRT confirms us that it is more like a bug and not a real security problem.

Sergio de los Santos

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