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標 題: FreeBSD Security Advisory FreeBSD-SA-02:32.pppd
發信站: NCTU CSIE FreeBSD Server (Wed Jul 31 20:45:34 2002)
轉信站: krynn!news.dragon2.net!FreeBSD.csie.NCTU!not-for-mail
Origin: freebsd.csie.nctu.edu.tw
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FreeBSD-SA-02:32.pppd Security Advisory
The FreeBSD Project
Topic: exploitable race condition in pppd
Category: core
Module: pppd
Announced: 2002-07-31
Credits: Sebastian Krahmer <[email protected]>
Affects: All releases of FreeBSD up to and including 4.6.1-RELEASE-p1
Corrected: 2002-07-30 03:50:40 UTC (RELENG_4)
2002-07-30 19:15:52 UTC (RELENG_4_6)
2002-07-30 19:16:46 UTC (RELENG_4_5)
2002-07-30 19:17:27 UTC (RELENG_4_4)
FreeBSD only: NO
I. Background
FreeBSD ships with several implementations of the Point-to-Point
Protocol (PPP). The pppd program is one of these implementations. It
provides basic support for negotiating a link, while encapsulation is
done by driver code in the kernel.
II. Problem Description
A race condition exists in the pppd program that may be exploited
in order to change the permissions of an arbitrary file. The file
specified as the tty device is opened by pppd, and the permissions
are recorded. If pppd fails to initialize the tty device in some way
(such as a failure of tcgetattr(3)), then pppd will then attempt to
restore the original permissions by calling chmod(2). The call to
chmod(2) is subject to a symlink race, so that the permissions may
`restored' on some other file.
Note that the pppd program is installed set-user-ID to root, so that
any file's permissions may be changed in this fashion.
III. Impact
A malicious local user may exploit the race condition to acquire write
permissions to a critical system file, such as /etc/crontab, and
leverage the situation to acquire escalated privileges.
In FreeBSD 4.4-RELEASE and later, the local user must be in group
`dialer' in order to run pppd and attempt to exploit this race.
IV. Workaround
Remove the set-user-ID bit from pppd by executing the following
command as root:
# chmod u-s /usr/sbin/pppd
V. Solution
Do one of the following:
1) Upgrade your vulnerable system to 4.6-STABLE; or to the RELENG_4_6,
RELENG_4_5, or RELENG_4_4 security branch dated after the correction
date (4.6.1-RELEASE-p2, 4.5-RELEASE-p11, or 4.4-RELEASE-p18).
2) To patch your present system:
The following patch has been verified to apply to FreeBSD 4.4, 4.5,
and 4.6 systems.
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:32/pppd.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:32/pppd.patch.asc
b) Execute the following commands as root:
# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/usr.sbin/pppd
# make depend && make && make install
VI. Correction details
The following list contains the revision numbers of each file that was
corrected in FreeBSD.
Path Revision
Branch
- -------------------------------------------------------------------------
usr.sbin/pppd/main.c
RELENG_4 1.19.2.1
RELENG_4_6 1.19.10.1
RELENG_4_5 1.19.8.1
RELENG_4_4 1.19.6.1
sys/conf/newvers.sh
RELENG_4_6 1.44.2.23.2.7
RELENG_4_5 1.44.2.20.2.12
RELENG_4_4 1.44.2.17.2.17
- -------------------------------------------------------------------------
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