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AUSCERT External Security Bulletin Redistribution
ESB-2001.067 -- FreeBSD-SA-01:25.kerberosIV
Local and remote vulnerabilities in Kerberos IV
19 February 2001
AusCERT Security Bulletin Summary
Operating System: FreeBSD
Impact: Execute Arbitrary Code/Commands
Access Required: Remote
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FreeBSD-SA-01:25 Security Advisory
Topic: Local and remote vulnerabilities in Kerberos IV
Module: libkrb, telnetd
Credits: Jouko Pynnönen <firstname.lastname@example.org>
Affects: FreeBSD 4.2-STABLE and 3.5-STABLE prior to the
Corrected: 2000-12-13 (FreeBSD 4.2-STABLE)
2000-12-15 (FreeBSD 3.5-STABLE)
FreeBSD only: NO
telnetd is the server for the telnet remote login protocol, which is
available with optional support for the Kerberos authentication
protocol. libkrb is the library used for Kerberised applications
(including telnetd and login). FreeBSD includes the KTH Kerberos
implementation, which is externally maintained, contributed software,
as an optional part of the base system.
II. Problem Description
The advisory describes three vulnerabilities: first, an overflow in
the libkrb KerberosIV authentication library, second, improper
filtering of environmental variables by the KerberosIV-adapted telnet
daemon, and finally, a temporary file vulnerability in the KerberosIV
ticket management code.
A buffer overflow exists in the libkrb Kerberos authentication
library, which may be exploitable by malicious remote authentication
servers. This vulnerability exists in the kdc_reply_cipher() call.
An attacker may be able to overflow this buffer during an
authentication exchange, allowing the attacker to execute arbitrary
code with the privileges of the caller of kdc_reply_cipher().
The telnet protocol allows for UNIX environmental variables to be
passed from the client to the user login session on the server. The
base system telnet daemon, telnetd, goes the great lengths to limit
the variables passed so as to prevent them from improperly influencing
the login and authentication mechanisms. The telnet daemon used with
KerberosIV relied on an incomplete list of improper environment
variables to remove from the environment before executing the login
program. This is a similar vulnerability to that described in
Security Advisory 00:69.
Two environment variables have been identified that place users of
Kerberos at risk. The first allows the remote user to change the
Kerberos server used for authentication requests, increasing the
opportunity for an attacker to exploit the buffer overflow. The
second allows the configuration directory for Kerberos to be modified,
allowing an attacker with the right to modify the local file system to
cause Kerberos to autheticate using an improper configuration
(including Kerberos realm and server configuration, as well as
srvtab). These vulnerabilities may be used to leverage root access.
A race condition exists in the handling of ticket files in /tmp; this
vulnerability may be exploited by a local user to gain ownership of
arbitrary files in the file system. This vulnerability can be
leveraged to gain root access.
These vulnerabilities only exist on systems which have installed the
optional Kerberos IV distribution (whether or not it is configured),
which is not installed by default.
If your system has the KerberosIV distribution installed, remote and
local users may be able to obtain root privileges on the local system.
To prevent remote root compromise via the telnet service, disable the
telnet service, which is usually run out of inetd: comment out the
following lines in /etc/inetd.conf, if present.
telnet stream tcp nowait root /usr/libexec/telnetd telnetd
telnet stream tcp6 nowait root /usr/libexec/telnetd telnetd
The local root compromise cannot be easily worked around.
One of the following:
1) Upgrade your vulnerable FreeBSD system to 4.2-STABLE or
3.5-STABLE after the respective correction dates.
2) Apply the relevant patch from below and recompile the affected
Download the relevant patch and detached PGP signature from the
following locations, and verify the signature using your PGP utility.
NOTE: This patch assumes you have already applied the patch in security advisory
Execute the following commands as root:
# cd /usr/src
# patch -p < /path/to/patch
# cd /usr/src/kerberosIV
# make depend && make all install
# cd /usr/src/libexec/telnetd
# make depend && make all install
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