Revised OpenSSH Security Advisory (adv.iss)

[Markus Friedl <markus () openbsd org>]

This is the 2nd 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.

        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];