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Re: Remote crash in tcpdump from OpenBSD
From: Balaram Amgoth <ramgoth () yahoo com>
Date: 6 Aug 2004 07:41:42 -0000

In-Reply-To: <20031221174837.14808.qmail () sf-www3-symnsj securityfocus com>

Hi Mike,
 Will the following packet be enough to reproduce this problem.

 char packet[] = "\x82\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00";

Thanks for your time in advance


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From: <mrh_tech () yahoo com>
To: bugtraq () securityfocus com
Subject: Re: Remote crash in tcpdump from OpenBSD

In-Reply-To: <3FE4CAC1.8010306 () freebsd lublin pl>

When an l2tp control packet is sent with optional bits set but containing invalid data, l2tp_avp_print() is passed 
this bad data.
Then, l2tp_avp_print() calls itself and continues an infinite loop of passing bad data to itself.

I had the consistent results sending:
\x82 (control+length bits)
\0x02 (version) then 10 bytes of zeros.

This is in print-l2tp.c
Lines: ~566-616

After commenting out (breaking the infinite loop):
~609: l2tp_avp_print(dat + len, length - len);
I was no longer able to crash tcpdump.

Obviously, properly validating the input is the real solution.

Tested on: OpenBSD 3.3 and 3.4 
tcpdump: 3.4.0
libpcap" 0.5


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