KRB5 TELNETD BUFFER OVERFLOWS

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             AUSCERT External Security Bulletin Redistribution

                      ESB-2001.330 -- MIT Kerberos 5
                       KRB5 TELNETD BUFFER OVERFLOWS
                               2 August 2001

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:                MIT Kerberos 5
Vendor:                 MIT
Impact:                 Execute Arbitrary Code/Commands
Access Required:        Remote

Ref:                    ESB-2001.312

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		    KRB5 TELNETD BUFFER OVERFLOWS

2001-07-31

SUMMARY:

Buffer overflows exist in the telnet daemon included with MIT krb5.
Exploits are believed to exist for various operating systems on at
least the i386 architecture.

IMPACT:

If telnetd is running, a remote user may gain unauthorized root
access.

VULNERABLE DISTRIBUTIONS:

* MIT Kerberos 5, all releases to date.

FIXES:

The recommended approach is to apply the appropriate patches and to
rebuild your telnetd.  Patches for the krb5-1.2.2 release may be found
at:

	http://web.mit.edu/kerberos/www/advisories/telnetd_122_patch.txt

The associated detached PGP signature is at:

	http://web.mit.edu/kerberos/www/advisories/telnetd_122_patch.txt.asc

These patches might apply successfully to older releases with some
amount of fuzz.

Please note that if you are using GNU make to build your krb5 sources,
the build system may attempt to rebuild the configure script from the
changed configure.in.  This may cause trouble if you don't have
autoconf installed properly.  To prevent this, you should use the
touch command or some similar means to ensure that the file
modification time on the configure script is newer than that of the
configure.in file.

If you are unable to patch your telnetd, you may should disable the
telnet service altogether.

This announcement and code patches related to it may be found on the
MIT Kerberos security advisory page at:

	http://web.mit.edu/kerberos/www/advisories/index.html

The main MIT Kerberos web page is at:

	http://web.mit.edu/kerberos/www/index.html

ACKNOWLEDGMENTS:

Thanks to TESO for the original alert / Bugtraq posting.

Thanks to Jeffrey Altman for assistance in developing these patches.

DETAILS:

A buffer overflow bug was discovered in telnet daemons derived from
BSD source code.  Since the telnet daemon in MIT krb5 uses code
largely derived originally from BSD sources, it too is vulnerable.

By carefully constructing a series of telnet options to send to a
telnet server, a remote attacker may exercise a bug relating to lack
of bounds-checking, causing an overflow of a fixed-size buffer.  This
overflow may possibly force the execution of malicious code.

It is not known how difficult this vulnerability is to exploit, since
the buffer is not on the stack.  Some discussion seems to indicate
that exploits exist for this vulnerability that are believed to work
against various operating systems for i386-based machines.  It is not
known whether these existing exploits have been successfully ported to
other processors.

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