how malware prevents programs from running

In today’s battle with malware, I learned a couple of interesting new places in the registry to check:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options

Place a key in here named after the file you want to prevent running, then place a STRING value under the key named “debugger”. Now, set the value of “debugger” to cmd, or some other relatively harmless executable that ignores its standard input – and presto, the application matching the keyname won’t run. BAD MALWARE. NO COOKIE.

Ironically, this is also quite useful for the GOOD guys keeping relatively clueless but persistent users from running things they really shouldn’t, like notorious P2P clients. For extra points, create a file C:\null.cmd or similar that simply exits, and use that as the “debugger” – they don’t even see anything happening at all, it just “doesn’t work”. This will probably frustrate them enough to desist, at least for a while… particularly given how used they probably are to the machine not working, if they’re that persistently malwaring it up in the first place.


Place a STRING value in here, and ditto above. (This is where GPO disallowing particular mutexes (I think it’s by mutex, not filename) to run takes effect.)

The More You Know…

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Jim Salter

Mercenary sysadmin, open source advocate, and frotzer of the jim-jam.

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