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TUCoPS :: Unix :: General :: m-095.txt

OpenSSH Challenge Response Vulnerabilities (CIAC M-095)


                       The U.S. Department of Energy
                   Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                   OpenSSH Challenge Response Vulnerabilities
                    [OpenSSH Security Advisory 2nd Revision]

June 27, 2002 19:00 GMT                                           Number M-095
[Revised 9 July 2002]
PROBLEM:       Two vulnerabilities exist in OpenSSH challenge response 
               handling code. The first vulnerability is an integer overflow 
               in the handling of the number of responses received during 
               challenge response authentication. The second vulnerability is 
               a buffer overflow involving the number of responses received 
               during challenge response authentication.
VERSIONS:      OpenSSH 2.3.1 through 3.3
PLATFORM:      See CERT's Security Advisory CA-2002-18 for vendor updates and 
DAMAGE:        The vulnerabilities can be exploited remotely and allow an 
               attacker to execute arbitrary code. These vulnerabilities may 
               also cause a denial-of-service attack. 
SOLUTION:      Upgrade to OpenSSH 3.4, apply the patches recommended by 
               OpenSSH, or see your platform vendor's particular 
VULNERABILITY  The risk is HIGH. A remote attacker can execute code of choice 
ASSESSMENT:    with the privilege of running the sshd, often root. 

[***** Start OpenSSH Security Advisory 2nd Revision *****]

Subject: Revised OpenSSH Security Advisory (adv.iss)

This is the 4th revision of the Advisory.

1. Versions affected:

        Serveral versions of OpenSSH's sshd between 2.3.1 and 3.3
        contain an input validation error that can result in an
        integer overflow and privilege escalation.

        All versions between 2.3.1 and 3.3 contain a bug in the
        PAMAuthenticationViaKbdInt code.

        All versions between 2.9.9 and 3.3 contain a bug in the
        ChallengeResponseAuthentication code.

        OpenSSH 3.4 and later are not affected.

        OpenSSH 3.2 and later prevent privilege escalation if
        UsePrivilegeSeparation is enabled in sshd_config.  OpenSSH
        3.3 enables UsePrivilegeSeparation by default.

        Although some earlier versions are not affected upgrading
        to OpenSSH 3.4 is recommended, because OpenSSH 3.4 adds
        checks for a class of potential bugs.

2. Impact:

        This bug can be exploited remotely if
		ChallengeResponseAuthentication is enabled in sshd_config.
		This option is enabled by default on OpenBSD and other

        Affected are at least systems supporting s/key over
        SSH protocol version 2 (OpenBSD, FreeBSD and NetBSD
        as well as other systems supporting s/key with SSH).
        Exploitablitly of systems using
	has not been verified.

3. Short-Term Solution:
        Disable ChallengeResponseAuthentication in sshd_config.


	Disable PAMAuthenticationViaKbdInt in sshd_config.

	Alternatively you can prevent privilege escalation
	if you enable UsePrivilegeSeparation in sshd_config.

4. Solution:

	Upgrade to OpenSSH 3.4 or apply the following patches.

5. Credits:




Index: auth2-chall.c
RCS file: /cvs/src/usr.bin/ssh/auth2-chall.c,v
retrieving revision 1.18
diff -u -r1.18 auth2-chall.c
--- auth2-chall.c	19 Jun 2002 00:27:55 -0000	1.18
+++ auth2-chall.c	26 Jun 2002 09:37:03 -0000
@@ -256,6 +256,8 @@
 	authctxt->postponed = 0;	/* reset */
 	nresp = packet_get_int();
+	if (nresp > 100)
+		fatal("input_userauth_info_response: nresp too big %u", nresp);
 	if (nresp > 0) {
 		response = xmalloc(nresp * sizeof(char*));
 		for (i = 0; i < nresp; i++)


Index: auth2-pam.c
RCS file: /var/cvs/openssh/auth2-pam.c,v
retrieving revision 1.12
diff -u -r1.12 auth2-pam.c
--- auth2-pam.c	22 Jan 2002 12:43:13 -0000	1.12
+++ auth2-pam.c	26 Jun 2002 10:12:31 -0000
@@ -140,6 +140,15 @@
 	nresp = packet_get_int();	/* Number of responses. */
 	debug("got %d responses", nresp);
+	if (nresp != context_pam2.num_expected)
+		fatal("%s: Received incorrect number of responses "
+		    "(expected %u, received %u)", __func__, nresp,
+		    context_pam2.num_expected);
+	if (nresp > 100)
+		fatal("%s: too many replies", __func__);
 	for (i = 0; i < nresp; i++) {
 		int j = context_pam2.prompts[i];

[***** End OpenSSH Security Advisory 2nd Revision *****]

CIAC wishes to acknowledge the contributions of OpenBSD for the 
information contained in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

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