Fix CVE-2018-15473, a method by which remote clients can enumerate
usernames on the server:

http://seclists.org/oss-sec/2018/q3/124
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-15473

Patch adapted from upstream source repository:

https://anongit.mindrot.org/openssh.git/commit/?id=74287f5df9966a0648b4a68417451dd18f079ab8

From 74287f5df9966a0648b4a68417451dd18f079ab8 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Tue, 31 Jul 2018 03:10:27 +0000
Subject: [PATCH] upstream: delay bailout for invalid authentic

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OpenBSD-Commit-ID: b4891882fbe413f230fe8ac8a37349b03bd0b70d
---
 auth2-gss.c       | 11 +++++++----
 auth2-hostbased.c | 11 ++++++-----
 auth2-pubkey.c    | 25 +++++++++++++++----------
 3 files changed, 28 insertions(+), 19 deletions(-)

# Adapted from upstream to apply to OpenSSH 7.7p1.
diff --git a/auth2-gss.c b/auth2-gss.c
index 589283b7..1d7cfb39 100644
--- a/auth2-gss.c
+++ b/auth2-gss.c
@@ -69,9 +69,6 @@ userauth_gssapi(struct ssh *ssh)
 	u_int len;
 	u_char *doid = NULL;
 
-	if (!authctxt->valid || authctxt->user == NULL)
-		return (0);
-
 	mechs = packet_get_int();
 	if (mechs == 0) {
 		debug("Mechanism negotiation is not supported");
diff --git a/auth2-gss.c b/auth2-gss.c
index 47308c5c..9351e042 100644
--- a/auth2-gss.c
+++ b/auth2-gss.c
@@ -106,6 +103,12 @@ userauth_gssapi(struct ssh *ssh)
 		return (0);
 	}
 
+	if (!authctxt->valid || authctxt->user == NULL) {
+		debug2("%s: disabled because of invalid user", __func__);
+		free(doid);
+		return (0);
+	}
+
 	if (GSS_ERROR(PRIVSEP(ssh_gssapi_server_ctx(&ctxt, &goid)))) {
 		if (ctxt != NULL)
 			ssh_gssapi_delete_ctx(&ctxt);
diff --git a/auth2-hostbased.c b/auth2-hostbased.c
index 60159a56..35939329 100644
--- a/auth2-hostbased.c
+++ b/auth2-hostbased.c
@@ -67,10 +67,6 @@ userauth_hostbased(struct ssh *ssh)
 	size_t alen, blen, slen;
 	int r, pktype, authenticated = 0;
 
-	if (!authctxt->valid) {
-		debug2("%s: disabled because of invalid user", __func__);
-		return 0;
-	}
 	/* XXX use sshkey_froms() */
 	if ((r = sshpkt_get_cstring(ssh, &pkalg, &alen)) != 0 ||
 	    (r = sshpkt_get_string(ssh, &pkblob, &blen)) != 0 ||
@@ -117,6 +113,11 @@ userauth_hostbased(struct ssh *ssh)
 		goto done;
 	}
 
+	if (!authctxt->valid || authctxt->user == NULL) {
+		debug2("%s: disabled because of invalid user", __func__);
+		goto done;
+	}
+
 	if ((b = sshbuf_new()) == NULL)
 		fatal("%s: sshbuf_new failed", __func__);
 	/* reconstruct packet */
diff --git a/auth2-pubkey.c b/auth2-pubkey.c
index c4d0f790..e1c15040 100644
--- a/auth2-pubkey.c
+++ b/auth2-pubkey.c
@@ -89,19 +89,15 @@ userauth_pubkey(struct ssh *ssh)
 {
 	Authctxt *authctxt = ssh->authctxt;
 	struct passwd *pw = authctxt->pw;
-	struct sshbuf *b;
+	struct sshbuf *b = NULL;
 	struct sshkey *key = NULL;
-	char *pkalg, *userstyle = NULL, *key_s = NULL, *ca_s = NULL;
-	u_char *pkblob, *sig, have_sig;
+	char *pkalg = NULL, *userstyle = NULL, *key_s = NULL, *ca_s = NULL;
+	u_char *pkblob = NULL, *sig = NULL, have_sig;
 	size_t blen, slen;
 	int r, pktype;
 	int authenticated = 0;
 	struct sshauthopt *authopts = NULL;
 
-	if (!authctxt->valid) {
-		debug2("%s: disabled because of invalid user", __func__);
-		return 0;
-	}
 	if ((r = sshpkt_get_u8(ssh, &have_sig)) != 0 ||
 	    (r = sshpkt_get_cstring(ssh, &pkalg, NULL)) != 0 ||
 	    (r = sshpkt_get_string(ssh, &pkblob, &blen)) != 0)
@@ -167,6 +163,11 @@ userauth_pubkey(struct ssh *ssh)
 				fatal("%s: sshbuf_put_string session id: %s",
 				    __func__, ssh_err(r));
 		}
+		if (!authctxt->valid || authctxt->user == NULL) {
+			debug2("%s: disabled because of invalid user",
+			    __func__);
+			goto done;
+		}
 		/* reconstruct packet */
 		xasprintf(&userstyle, "%s%s%s", authctxt->user,
 		    authctxt->style ? ":" : "",
@@ -183,7 +184,6 @@ userauth_pubkey(struct ssh *ssh)
 #ifdef DEBUG_PK
 		sshbuf_dump(b, stderr);
 #endif
-
 		/* test for correct signature */
 		authenticated = 0;
 		if (PRIVSEP(user_key_allowed(ssh, pw, key, 1, &authopts)) &&
@@ -194,7 +194,6 @@ userauth_pubkey(struct ssh *ssh)
 			authenticated = 1;
 		}
 		sshbuf_free(b);
-		free(sig);
 		auth2_record_key(authctxt, authenticated, key);
 	} else {
 		debug("%s: test pkalg %s pkblob %s%s%s",
@@ -205,6 +204,11 @@ userauth_pubkey(struct ssh *ssh)
 		if ((r = sshpkt_get_end(ssh)) != 0)
 			fatal("%s: %s", __func__, ssh_err(r));
 
+		if (!authctxt->valid || authctxt->user == NULL) {
+			debug2("%s: disabled because of invalid user",
+			    __func__);
+			goto done;
+		}
 		/* XXX fake reply and always send PK_OK ? */
 		/*
 		 * XXX this allows testing whether a user is allowed
@@ -238,6 +242,7 @@ done:
 	free(pkblob);
 	free(key_s);
 	free(ca_s);
+	free(sig);
 	return authenticated;
 }
 
-- 
2.18.0