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   HACK: Shell access users can use "popper" to create root owned files 
 System: Unix
 Source: Mark Fullmer (maf@cob.ohio-state.edu) from Bugtraq
   Date: Fri, 6 May 1994

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On systems that have /var/spool/mail mode 'drwsrwxrwt' and use the Berkeley
popper daemon, users that have access to /var/spool/mail (ie. a user with a
shell login) can create arbitrary root owned files on the host that popper
is executed on.Depending on the umask that popper was run with, this file may also be world
writeable.

Details: version UCB Pop server (version 1.831beta)

#line 59 of pop_dropcopy.c:
# currently running as root: (POP_TMPDROP is /usr/spool/mail/tmpXXXXXX)

>    strcpy(template,POP_TMPDROP);
>    (void) mktemp(template);

# The race starts.
# if a user guesses the pathname in "template", they could of previously
# made a link to say /etc/nologin.

# instead of script to exploit this bug, you can verify it exists by adding
# sleep(30) here -- after the mktemp(), before the fopen(), which will make the
# race condition easy to win.  Ofcourse, you allready need to be root to be
# able to do this...

>    if ( (tf=fopen(template,"w+")) == NULL ) {  /* failure, bail out    */
>        pop_log(p,POP_PRIORITY,
>            "Unable to create temporary temporary maildrop '%s': %s",template,
>                (errno < sys_nerr) ? sys_errlist[errno] : "") ;
>        return pop_msg(p,POP_FAILURE,
>        "System error, can't create temporary file.");
>    }

# at this point, the file is created.  Depending on the umask that popper was
# run with, this file may have world write permission.

# chown/chmod won't follow your link.
>    /* Now give this file to the user   */
>    (void) chown(template,pwp->pw_uid, pwp->pw_gid);]
>    (void) chmod(template,0600);
>       /* Now link this file to the temporary maildrop.  If this fails it
>     * is probably because the temporary maildrop already exists.  If so,
>     * this is ok.  We can just go on our way, because by the time we try
>     * to write into the file we will be running as the user.
>     */
>    (void) link(template,p->temp_drop);
>    (void) fclose(tf);
>    (void) unlink(template);

>    /* Now we run as the user. */
>    (void) setuid(pwp->pw_uid);
>    (void) setgid(pwp->pw_gid);

Solution.  If your /var/spool/mail is mode 'drwsrwxrwt' this code isn't
necessary.  Remove lines 59-82 of pop_dropcopy.c.  This doesn't entirely
solve the problem, especially if root reads their mail via popper.  The
best solution is to not have /var/spool/mail with world write permissions,
as this same type of problem exists in atleast one delivery agent (/bin/mail),
and probably in user agents.