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The chunked encoding hole and exploits

  • Buffer overflow in Apache 1.3.x prior to 1.3.26

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  • Also 2.x prior to 2.0.39

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  • Originally claimed not to be exploitable on x86, but a working exploit -- and a worm that attacks Apache on FreeBSD -- soon appeared

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  • If you're using 1.3, upgrading to 1.3.26 works.... However, when a server running 2.0.39 is attacked, worker processes/threads can hang, leading to resource exhaustion -- especially on systems that don't use threading (patch is already in the Apache source tree and the FreeBSD port)

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  • Installation of mod_blowchunks (for Apache 1.3.x only right now) recommended to block attacks and report infections

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  • Produces a message like the following:
[Sat Jun 22 17:42:47 2002] [error] [client X.X.X.X] Transfer-Encoding: 
chunked - denied and logged