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Subject: vixie cron...
From: Michal Zalewski (lcamtufTPI.PL)
Date: Thu Nov 16 2000 - 22:41:32 CST

Attached shell-script exploits fopen() + preserved umask vulnerability in
Paul Vixie's cron code. It will work on systems where /var/spool/cron is
user-readable (eg. 0755) - AFAIR Debian does so. RedHat (at least 6.1 and
previous) have mode 0700 on /var/spool/cron, and thus it isn't exploitable
in its default configuration... (ahmm, but this does NOT mean it is a
problem of o+rx bits, but of insecure umask() and fopen() calls).

I have no information about other distributions or systems - this exploit
should automagically detect if you are vulnerable or not (checking
/var/spool/cron, looking for Paul Vixie's crontab, etc). Please report
your findings to me and/or to BUGTRAQ.

If any of your users launched this exploit on screen, and then any other
user (including superuser) invoked "crontab -e" to change his/her crontab
entries, privledges elevation will occour. The main attack is performed
while root (or any other user, but this particular exploit is configured
against root - feel free to change it) is editing his crontab entry. After
any modification, when crontabs are updated, this exploit will try to
insert evil code over the original contents of the crontab file
(probability of successful exploitation is near to 100%). This, after
approximately one minute, leads to account compromise.

At the beginning, this exploit is trying to abuse crontab utility in order
to create somewhat enormous number of world-writable temporary files
(these files are open with fopen(), and then rename()d to destination name
- ugh!). It might take some time and cause less or more heavy load on
ancient boxes. After finishing it, exploit is waiting, consuming little or
no system resources, till "crontab -e" session will appear. For more
details, see exploit code.

Vendors were not notified because I have no idea which systems and distros
are shipping vulnerable configuration, and because pretty good workaround
is simple: chmod 700 /var/spool/cron.

Michal Zalewski [lcamtuftpi.pl] [tp.internet/security]
[http://lcamtuf.na.export.pl] <=--=> bash$ :(){ :|:&};:
=-----=> God is real, unless declared integer. <=-----=

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