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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 

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



http://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/scan_ncurses.sh

http://ftp.freebsd.org/pub/FreeBSD/CERT/tools/SA-00:68/test_ncurses.sh



e.g. with the fetch(1) command:



# fetch http://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 http://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 http://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:68/ncurses.tar.gz

# fetch http://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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