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Re: Remote crash in tcpdump from OpenBSD



On Fri, 6 Aug 2004, Balaram Amgoth wrote:

In-Reply-To: <20031221174837.14808.qmail@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>

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";

FYI, this is a report from December 2003. This has been fixed in OpenBSD
3.4-stable and (then -current, now) 3.5. Feel free to expore this further, but I don't think this is very interesting material for this list, more than 7 months after the original report.

-Otto


Thanks for your time in advance

Balaram



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From: <mrh_tech@xxxxxxxxx>
To: bugtraq@xxxxxxxxxxxxxxxxx
Subject: Re: Remote crash in tcpdump from OpenBSD

In-Reply-To: <3FE4CAC1.8010306@xxxxxxxxxxxxxxxxx>

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

Thanks,
Mike