December 03, 2014

MITM watch - patching binaries at Tor exit nodes

The real MITMs are so rare that protocols that are designed around them fall to the Bayesian impossibility syndrome (*). In short, false negatives cause the system to be ignored, and when the real negative indicator turns up it is treated as a false. Ignored. Fail.

Here's some evidence of that with Tor:

... I tested BDFProxy against a number of binaries and update processes, including Microsoft Windows Automatic updates. The good news is that if an entity is actively patching Windows PE files for Windows Update, the update verification process detects it, and you will receive error code 0x80200053.

.... If you Google the error code, the official Microsoft response is troublesome.

If you follow the three steps from the official MS answer, two of those steps result in downloading and executing a MS 'Fixit' solution executable. ... If an adversary is currently patching binaries as you download them, these 'Fixit' executables will also be patched. Since the user, not the automatic update process, is initiating these downloads, these files are not automatically verified before execution as with Windows Update. In addition, these files need administrative privileges to execute, and they will execute the payload that was patched into the binary during download with those elevated privileges.

And, tomorrow, another MITM!

(*) I'd love to hear a better name than Bayesian impossibility syndrome, which I just made up. It's pretty important, it explains why the current SSL/PKI/CA MITM protection can never work, relying on Bayesian statistics to explain why infrequent real attacks cannot be defended against when overshadowed by frequent false negatives.

Posted by iang at December 3, 2014 09:40 AM
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