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FreeBSD-SA-03:01.cvs                                        Security Advisory
                                                          The FreeBSD Project

Topic:          remotely exploitable vulnerability in cvs server

Category:       contrib
Module:         contrib_cvs
Announced:      2003-02-04
Credits:        Stefan Esser <s.esser@e-matters.de>
Affects:        All FreeBSD versions prior to 4.6-RELEASE-p7, 4.7-RELEASE-p4,
                5.0-RELEASE-p1
Corrected:      2003-01-21 22:26:46 UTC (RELENG_4)
                2003-02-04 18:05:07 UTC (RELENG_5_0)
                2003-02-04 18:07:20 UTC (RELENG_4_7)
                2003-02-04 18:08:26 UTC (RELENG_4_6)
FreeBSD only:   NO

I.   Background

The Concurrent Versions System (CVS) is a version control system.  It
may be used to access a repository locally, or to access a `remote
repository' using several different methods, including `ext' (rsh),
and `pserver' (password-authenticated server).  When accessing a
remote repository, the target machine runs the CVS server to fulfill
client requests.

II.  Problem Description

The implementation of the CVS server contains a programming error which
can lead to a block of memory being freed more than once (i.e. a
double-free bug).

Separately, the CVS server allows clients with write access to specify
arbitrary commands to execute as part of an update (update-prog) or
commit (checkin-prog).  This is a dangerous feature that is generally
not needed: there are other, safer methods of triggering program
execution.

III. Impact

An attacker may exploit the double-free bug in order to bypass write
access checks.  Combined with the update-prog/checkin-prog feature,
the attacker may be able to execute arbitrary commands with the
privileges of the CVS server.  The impact is most severe when running
the CVS server in `pserver' mode to provide read-only access to the
world (anoncvs).

IV.  Workaround

Do not use `pserver' mode directly.  Instead, use one of the safer
methods described in the following online resources:

<URL:http://openbsd.sunsite.ualberta.ca/papers/anoncvs-paper.ps>
<URL:http://www.netsys.com/library/papers/chrooted-ssh-cvs-server.txt>

V.   Solution

1) Upgrade your vulnerable system to 4.7-STABLE; or to the RELENG_4_7
(4.7-RELEASE-p4), RELENG_4_6 (4.6-RELEASE-p7), or RELENG_5_0
(5.0-RELEASE-p1) security branch dated after the correction date.

2) To patch your present system:

The following patch has been verified to apply to FreeBSD 4.6, 4.7, and
5.0 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-03:01/cvs.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-03:01/cvs.patch.asc

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/gnu/usr.bin/cvs
# make obj && 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
- -------------------------------------------------------------------------
src/contrib/cvs/src/server.c
  RELENG_5_0                                                     1.17.2.1
  RELENG_4_7                                                 1.13.2.2.6.1
  RELENG_4_6                                                 1.13.2.2.4.1
- -------------------------------------------------------------------------

VII. References

<URL:http://online.securityfocus.com/archive/1/72584>
<URL:http://security.e-matters.de/advisories/012003.html>
<URL:http://ccvs.cvshome.org/servlets/NewsItemView?newsID=51>
<URL:http://www.kb.cert.org/vuls/id/650937>
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