MITKRB5-SA-2011-008 buffer overflow in telnetd [CVE-2011-4862]

[Tom Yu]

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MITKRB5-SA-2011-008

MIT krb5 Security Advisory 2011-008
Original release: 2011-12-26
Last update: 2011-12-26

Topic: buffer overflow in telnetd

CVE-2011-4862

CVSSv2 Vector: AV:N/AC:L/Au:N/C:C/I:C/A:C/E:F/RL:OF/RC:C

CVSSv2 Base Score:      10

Access Vector:          Network
Access Complexity:      Low
Authentication:         None
Confidentiality Impact: Complete
Integrity Impact:       Complete
Availability Impact:    Complete

CVSSv2 Temporal Score:  8.3

Exploitability:         Functional
Remediation Level:      Official Fix
Report Confidence:      Confirmed

SUMMARY
=======

The telnet daemon (telnetd) in MIT krb5 (and in krb5-appl after the
applications were moved to a separate distribution for krb5-1.8) is
vulnerable to a buffer overflow.  The flaw does not require
authentication to exploit.  Exploit code is reported to be actively
used in the wild.

IMPACT
======

An unauthenticated remote attacker can cause a buffer overflow and
probably execute arbitrary code with the privileges of the telnet
daemon (normally root).

AFFECTED SOFTWARE
=================

* The telnet daemon in all releases of MIT krb5 prior to krb5-1.8 is
  vulnerable.  Later releases moved the telnet code to the krb5-appl
  distribution.

* The telnet daemon in all releases of krb5-appl is vulnerable.

FIXES
=====

* Workaround: Disable telnet and use a more secure remote login
  solution, such as SSH.

* A future release of krb5-appl will fix this vulnerability.

* Apply the following patch:

diff --git a/telnet/libtelnet/encrypt.c b/telnet/libtelnet/encrypt.c
index f75317d..b8d6cdd 100644
- --- a/telnet/libtelnet/encrypt.c
+++ b/telnet/libtelnet/encrypt.c
@@ -757,6 +757,9 @@ static void encrypt_keyid(kp, keyid, len)
 	int dir = kp->dir;
 	register int ret = 0;
 
+	if (len > MAXKEYLEN)
+		len = MAXKEYLEN;
+
 	if (!(ep = (*kp->getcrypt)(*kp->modep))) {
 		if (len == 0)
 			return;


  This patch is also available at

  http://web.mit.edu/kerberos/advisories/2011-008-patch.txt

  A PGP-signed patch is available at

  http://web.mit.edu/kerberos/advisories/2011-008-patch.txt.asc

REFERENCES
==========

This announcement is posted at:

  http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2011-008.txt

This announcement and related security advisories may be found on the
MIT Kerberos security advisory page at:

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

The main MIT Kerberos web page is at:

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

CVSSv2:

    http://www.first.org/cvss/cvss-guide.html
    http://nvd.nist.gov/cvss.cfm?calculator&adv&version=2

CVE: CVE-2011-4862
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-4862

http://lists.freebsd.org/pipermail/freebsd-security/2011-December/006117.html

ACKNOWLEDGMENTS
===============

We became aware of this vulnerability through a FreeBSD security
advisory.

CONTACT
=======

The MIT Kerberos Team security contact address is
<krbcore-security at mit.edu>.  When sending sensitive information,
please PGP-encrypt it using the following key:

pub   2048R/56CD8F76 2010-12-29 [expires: 2012-02-01]
uid     MIT Kerberos Team Security Contact <krbcore-security at mit.edu>

DETAILS
=======

If the telnetd receives an ENCRYPT suboption that includes a key ID,
encrypt_keyid() in libtelnet/encrypt.c copies the suboption contents
into a fixed-size static buffer without first constraining the length,
leading to a buffer overflow.

REVISION HISTORY
================

2011-12-26      original release

Copyright (C) 2011 Massachusetts Institute of Technology
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