FreeBSD Security Advisory FreeBSD-SA-06:09.openssh [REVISED]

2006-03-01T00:00:00
ID SECURITYVULNS:DOC:11639
Type securityvulns
Reporter Securityvulns
Modified 2006-03-01T00:00:00

Description

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

============================================================================= FreeBSD-SA-06:09.openssh Security Advisory The FreeBSD Project

Topic: Remote denial of service in OpenSSH

Category: contrib Module: OpenSSH Announced: 2006-03-01 Affects: FreeBSD 5.3 and 5.4 Corrected: 2006-03-01 14:19:48 UTC (RELENG_5, 5.5-PRERELEASE) 2006-03-01 14:21:01 UTC (RELENG_5_4, 5.4-RELEASE-p12) 2006-03-01 14:24:52 UTC (RELENG_5_3, 5.3-RELEASE-p27) CVE Name: CVE-2006-0883

For general information regarding FreeBSD Security Advisories, including descriptions of the fields above, security branches, and the following sections, please visit <URL:http://www.freebsd.org/security/>.

  1. Revision History

v1.0 2006-03-01 Initial release. v1.1 2006-03-01 Corrected workaround instructions.

I. Background

OpenSSH is an implementation of the SSH protocol suite, providing an encrypted, authenticated transport for a variety of services, including remote shell access.

Privilege separation is a mechanism used by OpenSSH to protect itself against possible future vulnerabilities. It works by splitting the server process in two: the child process drops its privileges and carries on the conversation with the client, while the parent retains its privileges, monitors the child, and performs privileged operations on behalf of the child when it is satisified that everything is in order. Privilege separation is enabled by default in FreeBSD.

OpenPAM is an implementation of the PAM framework, which allows the use of loadable modules to implement user authentication and session management in a manner defined by the administrator. It is used by OpenSSH and numerous other applications in FreeBSD to provide a consistent and configurable authentication system.

II. Problem Description

Because OpenSSH and OpenPAM have conflicting designs (one is event- driven while the other is callback-driven), it is necessary for OpenSSH to fork a child process to handle calls to the PAM framework. However, if the unprivileged child terminates while PAM authentication is under way, the parent process incorrectly believes that the PAM child also terminated. The parent process then terminates, and the PAM child is left behind.

Due to the way OpenSSH performs internal accounting, these orphaned PAM children are counted as pending connections by the master OpenSSH server process. Once a certain number of orphans has accumulated, the master decides that it is overloaded and stops accepting client connections.

III. Impact

By repeatedly connecting to a vulnerable server, waiting for a password prompt, and closing the connection, an attacker can cause OpenSSH to stop accepting client connections until the system restarts or an administrator manually kills the orphaned PAM processes.

IV. Workaround

The following command will show a list of orphaned PAM processes:

pgrep -lf 'sshd.*\[pam\]'

The following command will kill orphaned PAM processes:

pkill -f 'sshd.*\[pam\]'

To prevent OpenSSH from leaving orphaned PAM processes behind, perform one of the following:

1) Disable PAM authentication in OpenSSH. Users will still be able to log in using their Unix password, OPIE or SSH keys.

To do this, execute the following commands as root:

echo 'UsePAM no' >>/etc/ssh/sshd_config

/etc/rc.d/sshd restart

2) If disabling PAM is not an option - if, for instance, you use RADIUS authentication, or store user passwords in an SQL database - you may instead disable privilege separation. However, this may leave OpenSSH vulnerable to hitherto unknown bugs, and should be considered a last resort.

To do this, execute the following commands as root:

echo 'UsePrivilegeSeparation no' >>/etc/ssh/sshd_config

/etc/rc.d/sshd restart

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to 5-STABLE or to the RELENG_5_4 or RELENG_5_3 security branch dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 5.3 and 5.4 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-06:09/openssh.patch

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:09/openssh.patch.asc

b) Execute the following commands as root:

cd /usr/src

patch < /path/to/patch

cd /usr/src/secure/lib/libssh

make obj && make depend && make && make install

cd /usr/src/secure/usr.sbin/sshd

make obj && make depend && make && make install

/etc/rc.d/sshd restart

VI. Correction details

The following list contains the revision numbers of each file that was corrected in FreeBSD.

Branch Revision Path


RELENG_5 src/crypto/openssh/auth-pam.c 1.14.2.1 src/crypto/openssh/ssh_config 1.25.2.1 src/crypto/openssh/ssh_config.5 1.15.2.1 src/crypto/openssh/sshd_config 1.40.2.1 src/crypto/openssh/sshd_config.5 1.21.2.1 src/crypto/openssh/version.h 1.27.2.1 RELENG_5_4 src/UPDATING 1.342.2.24.2.21 src/sys/conf/newvers.sh 1.62.2.18.2.17 src/crypto/openssh/auth-pam.c 1.14.6.1 src/crypto/openssh/ssh_config 1.25.6.1 src/crypto/openssh/ssh_config.5 1.15.6.1 src/crypto/openssh/sshd_config 1.40.6.1 src/crypto/openssh/sshd_config.5 1.21.6.1 src/crypto/openssh/version.h 1.27.6.1 RELENG_5_3 src/UPDATING 1.342.2.13.2.30 src/sys/conf/newvers.sh 1.62.2.15.2.32 src/crypto/openssh/auth-pam.c 1.14.4.1 src/crypto/openssh/ssh_config 1.25.4.1 src/crypto/openssh/ssh_config.5 1.15.4.1 src/crypto/openssh/sshd_config 1.40.4.1 src/crypto/openssh/sshd_config.5 1.21.4.1 src/crypto/openssh/version.h 1.27.4.1


VII. References

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=248125 http://bugzilla.mindrot.org/show_bug.cgi?id=839 http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0883

The latest revision of this advisory is available at ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-06:09.openssh.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (FreeBSD)

iD8DBQFEBcVoFdaIBMps37IRAtciAJ0ZEisuqx4lAtn2vr+44e4mrWYQuQCeLLRB kUuFQBE4Jy3SJeI+07wnknA= =esdm -----END PGP SIGNATURE-----