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FreeBSD-SA-00:68 Security Advisory
FreeBSD, Inc.
Topic: ncurses allows local privilege escalation
Category: core
Module: ncurses
Announced: 2000-11-13
Affects: FreeBSD 5.0-CURRENT, 4.x prior to the correction date.
FreeBSD 3.x vulnerability status currently unconfirmed.
Corrected: 2000-10-11 (FreeBSD 4.1.1-STABLE)
Credits: Jouko Pynnonen <This email address is being protected from spambots. You need JavaScript enabled to view it. >
FreeBSD only: NO
I. Background
ncurses is a text-mode display library used for formatting the output
of applications on a variety of terminals. It is externally
maintained, contributed code which is included in FreeBSD by default.
II. Problem Description
There exists an overflowable buffer in the libncurses library in the
processing of cursor movement capabilities. An attacker can force a
privileged application to use the attacker's termcap file containing a
specially crafted terminal entry, which will trigger the vulnerability
when the vulnerable ncurses code is called. This allows them to
execute arbitrary code on the local system with the privileges of the
exploited binary.
The systat utility included in the FreeBSD base system is known to use
vulnerable ncurses routines. It 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.
There may be other vulnerable applications included in the FreeBSD 4.x
base system, but no others are confirmed to be vulnerable due to the
difficulty in identifying a complete list of vulnerable ncurses
functions. However the following is a complete list of FreeBSD system
binaries which link against ncurses and run with increased
privileges. They may or may not be vulnerable to exploitation.
/usr/sbin/lpc
/usr/bin/top
/usr/bin/systat
FreeBSD 3.x and earlier versions use a very old, customized version of
ncurses which is difficult to update without breaking
backwards-compatibility. The update was made for FreeBSD 4.0, but 3.x
will not be updated to the newer version. At this stage the
vulnerability has not been confirmed in FreeBSD 3.x.
III. Impact
Certain setuid/setgid software (including FreeBSD base system
utilities and third party ports/packages) may be vulnerable to a local
exploit yielding privileged access.
The /usr/bin/systat utility is known to be vulnerable to this problem
in ncurses. At this time is unknown whether /usr/bin/top and
/usr/sbin/lpc are also affected.
The problems were corrected prior to the release of FreeBSD 4.2.
IV. Workaround
It is not feasible to reliably detect binaries which are vulnerable to
the ncurses vulnerability, however the provided utility will scan for
privileged binaries which use ncurses and which may potentially be
vulnerable. Some of the binaries reported may not in fact be
vulnerable, but should be recompiled anyway for maximum assurance of
security.
Statically linked binaries which are identified as potentially
vulnerable should be recompiled from source code if possible, after
patching and recompiling libncurses, in order to correct the
vulnerability. Dynamically linked binaries will be corrected by
simply patching and recompiling libncurses as described below.
As an interim measure, consider removing any identified setuid or
setgid binary, removing set[ug]id privileges from the file, or
limiting the file access permissions, as appropriate.
Of course, it is possible that some of the identified files may be
required for the correct operation of your local system, in which case
there is no clear workaround except for limiting the set of users who
may run the binaries, by an appropriate use of user groups and
removing the "o+x" file permission bit.
1) Download the 'scan_ncurses.sh' and 'test_ncurses.sh' scripts from
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/scan_ncurses.sh
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/test_ncurses.sh
e.g. with the fetch(1) command:
# fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/scan_ncurses.sh
Receiving scan_ncurses.sh (381 bytes): 100%
381 bytes transferred in 0.1 seconds (7.03 kBps)
# fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/test_ncurses.sh
Receiving test_ncurses.sh (604 bytes): 100%
604 bytes transferred in 0.1 seconds (6.55 kBps)
2) Verify the md5 checksums and compare to the value below:
# md5 scan_ncurses.sh
MD5 (scan_ncurses.sh) = 597f63af701253f053581aa1821cbac1
# md5 test_ncurses.sh
MD5 (test_ncurses.sh) = 12491ceb15415df7682e3797de53223e
3) Run the scan_ncurses.sh script against your system:
# chmod a+x ./test_ncurses.sh
# sh scan_ncurses.sh ./test_ncurses.sh /
This will scan your entire system for setuid or setgid binaries which
make use of the ncurses library. Each returned binary should be
examined (e.g. with 'ls -l' and/or other tools) to determine what
security risk it poses to your local environment, e.g. whether it can
be run by arbitrary local users who may be able to exploit it to gain
privileges.
4) Remove the binaries, or reduce their file permissions, as appropriate.
V. Solution
Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE after the
correction date, or patch your present system source code and
rebuild. Then run the scan_ncurses.sh script as instructed in section
IV and identify any statically-linked binaries as reported by the
script. These should either be removed, recompiled, or have privileges
restricted to secure them against this vulnerability (since
statically-linked binaries will not be affected by simply recompiling
the shared libncurses library).
To patch your present system: download the updated ncurses code from
the below location, and execute the following commands as root:
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:68/ncurses.tar.gz
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:68/ncurses.tar.gz.asc
Verify the detached PGP signature using your PGP utility.
cd /usr/src
tar xvfz /path/to/ncurses.tar.gz
cd /usr/src/lib/libncurses
make all
make install
In contrast to the usual practise, a simple patch fixing the security
vulnerability is not provided because the vendor did not make one
available, and the updated ncurses snapshot which fixed the
vulnerability contains numerous other changes whose purpose and
relation to the fix was unclear.
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