__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Capability
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__________________________________________________________
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.
AFFECTED
VERSIONS: OpenSSH 2.3.1 through 3.3
PLATFORM: See CERT's Security Advisory CA-2002-18 for vendor updates and
recommendations.
http://www.cert.org/advisories/CA-2002-18.html
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
recommendations.
______________________________________________________________________________
VULNERABILITY The risk is HIGH. A remote attacker can execute code of choice
ASSESSMENT: with the privilege of running the sshd, often root.
______________________________________________________________________________
LINKS:
CIAC BULLETIN: http://www.ciac.org/ciac/bulletins/m-095.shtml
ORIGINAL BULLETIN: http://openssh.org/txt/preauth.adv
CERT BULLETIN: http://www.cert.org/advisories/CA-2002-18.html
______________________________________________________________________________
[***** 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
systems.
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
PAMAuthenticationViaKbdInt
has not been verified.
3. Short-Term Solution:
Disable ChallengeResponseAuthentication in sshd_config.
and
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:
ISS.
Appendix:
A:
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++)
B:
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
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