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MS silently fixes password sniffing bug with XP SP1

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Keystrokes, including passwords, can be sniffed when using Windows Terminal Server or the XP remote control feature. MS has rolled a fix silently into SP1 without making any public statement on this serious problem.

The cause of the keystroke
-sniffing feature is a design mistake in Microsoft's Remote Desktop Protocol (RDP) which leaks information about the contents of encrypted packets through their checksums. This is because packets with the same plaintext have matching checksums throughout a particular session.

So if an attacker captures network traffic from an encrypted session it is possible to work out the keystrokes made by a user through rudimentary frequency analysis.

This hole was discovered by Ben Cohen of security developer Skygate, who reported it to MS on April 16. The issue is explained in greater detail on a posting to BugTraq made earlier this week.

This RDP flaw is not an easy vulnerability to exploit and can only be performed locally, Pete Chown, Skygate's director of security development, told us. But still it's a serious bug.

Skygate's Cohen has also discovered a scarcely less serious remote denial of service attack mechanism, involving XP Professional's Remote Desktop and RDP.

Here, it's possible to crash a client at the start of a session by sending commands, related to rendering patterns, which force a reboot (as explained here). Once again Skygate notified MS of the bug on April 16, and once again a fix was silently rolled into XP SP1.

To fix the vulns, you have two choices. You can install XP's new SP1, which will give Billg remote root privileges on your box by virtue of his new Trojan EULA (and silently re-enable some services you may have disable such as 'automatic update').

Or you can follow Skgate's suggested workarounds: go back to Microsoft Terminal Services Client version 4.0 rather than 5.0 to fix the keystroke-sniffing flaw; and disable Remote Desktop to remove the DoS risk.

Chown criticises Microsoft for its response to Skygate's bug reports. "Microsoft took a ridiculously long time to respond to the reports; we first notified them in April."

"They always gave the impression that fixing the problem was a long way down their priority list. Eventually a partial fix was slipped into XP Service Pack 1, without telling us and without telling anyone else that they are vulnerable."

By failing to issue an alert and, after a long delay, fixing flaws in the background, MS does a disservice to its customers.

First with the Win-XP Help Center problem (where requests could be used to silently delete the contents of Windows directories, and now with the RDP flaws, Redmond in effect ensures that any XP user who does not install SP1, and who is unaware of the flaw, will remain vulnerable.

Trustworthy Computing? ®

External Links

Vulnerability notice on the keystroke sniffing flaw and related discussion on SecurityFocus/BugTraq
Discussion of the Remote Desktop DoS vulnerability

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