FreeBSD Security Advisory: [REISSUED]

FreeBSD Security Advisories security-advisories at
Mon Nov 6 12:01:31 PST 2000


FreeBSD-SA-00:62                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          top allows reading of kernel memory [REISSUED]

Category:       core
Module:         top
Announced:      2000-11-01
Reissued:       2000-11-06
Credits:        vort at via OpenBSD
Affects:        FreeBSD 3.x (all releases), FreeBSD 4.x (all releases prior
                to 4.2), FreeBSD 3.5.1-STABLE and 4.1.1-STABLE prior
                to the correction date.
Corrected:      2000-11-04 (FreeBSD 4.1.1-STABLE)
                2000-11-05 (FreeBSD 3.5.1-STABLE)
FreeBSD only:   NO

0.   Revision History

v1.0  2000-11-01  Initial release
v1.1  2000-11-06  Updated patch released.

I.   Background

top is a utility for displaying current system resource statistics
such as process CPU and memory use.  It is externally-maintained,
contributed software which is included in FreeBSD by default.

II.  Problem Description

A "format string vulnerability" was discovered in the top(1) utility
which allows unprivileged local users to cause the top process to
execute arbitrary code.  The top utility runs with increased
privileges as a member of the kmem group, which allows it to read from
kernel memory (but not write to it).  A process with the ability to
read from kernel memory can monitor privileged data such as network
traffic, disk buffers and terminal activity, and may be able to
leverage this to obtain further privileges on the local system or on
other systems, including root privileges.

All released versions of FreeBSD prior to the correction date
including 4.0, 4.1, 4.1.1 and 3.5.1 are vulnerable to this problem,
but it was fixed in the 4.1.1-STABLE branch prior to the release of

III. Impact

Local users can read privileged data from kernel memory which may
provide information allowing them to further increase their local or
remote system access privileges.

IV.  Workaround

Remove the setgid bit on the top utilities.  This has the side-effect
that users who are not a member of the kmem group or who are not the
superuser cannot use the top utility.

# chmod g-s /usr/bin/top

V.   Solution

One of the following:

1) Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE or
3.5.1-STABLE after the respective correction dates.

2) Apply the patch below and recompile the relevant files:

NOTE: The original version of this advisory contained an incomplete
patch which does not fully eliminate the security vulnerability. The
additional vulnerability was pointed out by Przemyslaw Frasunek
<venglin at>.

Either save this advisory to a file, or download the patch and
detached PGP signature from the following locations, and verify the
signature using your PGP utility.

Execute the following commands as root:

# cd /usr/src/contrib/top
# patch -p < /path/to/patch_or_advisory
# cd /usr/src/usr.bin/top
# make depend && make all install

Patch for vulnerable systems:

    Index: display.c
    RCS file: /mnt/ncvs/src/contrib/top/display.c,v
    retrieving revision 1.4
    retrieving revision 1.5
    diff -u -r1.4 -r1.5
    --- display.c	1999/01/09 20:20:33	1.4
    +++ display.c	2000/10/04 23:34:16	1.5
    @@ -829,7 +831,7 @@
         register int i;
         /* first, format the message */
    -    (void) sprintf(next_msg, msgfmt, a1, a2, a3);
    +    (void) snprintf(next_msg, sizeof(next_msg), msgfmt, a1, a2, a3);
         if (msglen > 0)
    Index: top.c
    RCS file: /mnt/ncvs/src/contrib/top/top.c,v
    retrieving revision 1.4
    retrieving revision 1.5
    diff -u -r1.4 -r1.5
    --- top.c	1999/01/09 20:20:34	1.4
    +++ top.c	2000/10/04 23:34:16	1.5
    @@ -807,7 +809,7 @@
     				    if ((errmsg = kill_procs(tempbuf2)) != NULL)
    -					new_message(MT_standout, errmsg);
    +					new_message(MT_standout, "%s", errmsg);
     					no_command = Yes;
    Index: top.c
    RCS file: /mnt/ncvs/src/contrib/top/top.c,v
    retrieving revision 1.5
    retrieving revision 1.6
    diff -u -r1.5 -r1.6
    --- top.c	2000/10/04 23:34:16	1.5
    +++ top.c	2000/11/03 22:00:10	1.6
    @@ -826,7 +826,7 @@
     				    if ((errmsg = renice_procs(tempbuf2)) != NULL)
    -					new_message(MT_standout, errmsg);
    +					new_message(MT_standout, "%s", errmsg);
     					no_command = Yes;

Version: GnuPG v1.0.4 (FreeBSD)
Comment: For info see


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