OpenSSH security advisory: legacy certificate signing in 5.6/5.7

[Damien Miller]

OpenSSH Security Advisory: legacy-certs.adv

This document may be found at: http://www.openssh.com/txt/legacy-cert.adv

1. Vulnerability

        Legacy certificates generated by OpenSSH might contain data
        from the stack thus leaking confidential information.

2. Affected configurations

        OpenSSH 5.6 and OpenSSH 5.7 only when generating legacy
        certificates. These must be specifically requested using the
        "-t" option on the ssh-keygen CA command-line.

3. Mitigation

        Avoid generating legacy certificates using OpenSSH 5.6 or 5.7

        If legacy certificates have been issued with a vulnerable
        OpenSSH version, consider rotating any CA key used.

4. Details

        When generating legacy *-cert-v00 at openssh.com certificates,
        the nonce field was not being correctly filled with random
        data but was left uninitialised, containing the contents of
        the stack.

        The contents of the stack at this point in ssh-keygen's
        execution do not appear to leak the CA private key or other
        sensitive data, but this possibility cannot be excluded on
        all platforms and library versions.

        If certificates are generated using user-specified contents
        (as opposed to the CA specifying all fields) then they will
        be less resistant to hash collision attacks. Fortunately,
        such attacks are not currently considered practical for the
        SHA family of hashes used to sign these certificates.

5. Credit

        This issue was privately reported by Mateusz Kocielski on
        January 26, 2011.

6. Fix

        OpenSSH 5.8 contains a fix for this vulnerability. Users who
        prefer to continue to use OpenSSH 5.6 or 5.7 may apply this
        patch:

Index: key.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/key.c,v
retrieving revision 1.95
diff -u -r1.95 key.c
--- key.c	10 Nov 2010 01:33:07 -0000	1.95
+++ key.c	3 Feb 2011 06:52:33 -0000
@@ -1823,8 +1823,8 @@
 	buffer_put_cstring(&k->cert->certblob, key_ssh_name(k));
 
 	/* -v01 certs put nonce first */
+	arc4random_buf(&nonce, sizeof(nonce));
 	if (!key_cert_is_legacy(k)) {
-		arc4random_buf(&nonce, sizeof(nonce));
 		buffer_put_string(&k->cert->certblob, nonce, sizeof(nonce));
 	}