Bug#906236: openssh: CVE-2018-15473: delay bailout for invalid authenticating user until after the packet

[Chris Lamb <lamby@debian.org>]

Hi,

> openssh: CVE-2018-15473: delay bailout for invalid authenticating
> user until after the packet

I've started on a patch for wheezy (WIP attached).

Would the security team be interested in one for stretch? If so, I can
return with a proposed debdiff.


Regards,

-- 
      ,''`.
     : :'  :     Chris Lamb
     `. `'`      lamby@debian.org / chris-lamb.co.uk
       `-

--- openssh-6.7p1.orig/auth2-gss.c
+++ openssh-6.7p1/auth2-gss.c
@@ -102,9 +102,6 @@ userauth_gssapi(Authctxt *authctxt)
 	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");
@@ -135,6 +132,12 @@ userauth_gssapi(Authctxt *authctxt)
 		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);
--- openssh-6.7p1.orig/auth2-hostbased.c
+++ openssh-6.7p1/auth2-hostbased.c
@@ -65,10 +65,6 @@ userauth_hostbased(Authctxt *authctxt)
 	int pktype;
 	int authenticated = 0;
 
-	if (!authctxt->valid) {
-		debug2("userauth_hostbased: disabled because of invalid user");
-		return 0;
-	}
 	pkalg = packet_get_string(&alen);
 	pkblob = packet_get_string(&blen);
 	chost = packet_get_string(NULL);
@@ -107,6 +103,11 @@ userauth_hostbased(Authctxt *authctxt)
 		    "signature format");
 		goto done;
 	}
+	if (!authctxt->valid || authctxt->user == NULL) {
+		debug2("%s: disabled because of invalid user", __func__);
+		goto done;
+	}
+
 	service = datafellows & SSH_BUG_HBSERVICE ? "ssh-userauth" :
 	    authctxt->service;
 	buffer_init(&b);
--- openssh-6.7p1.orig/auth2-pubkey.c
+++ openssh-6.7p1/auth2-pubkey.c
@@ -76,15 +76,11 @@ userauth_pubkey(Authctxt *authctxt)
 	Buffer b;
 	Key *key = NULL;
 	char *pkalg, *userstyle;
-	u_char *pkblob, *sig;
+	u_char *pkblob, *sig = NULL;
 	u_int alen, blen, slen;
 	int have_sig, pktype;
 	int authenticated = 0;
 
-	if (!authctxt->valid) {
-		debug2("userauth_pubkey: disabled because of invalid user");
-		return 0;
-	}
 	have_sig = packet_get_char();
 	if (datafellows & SSH_BUG_PKAUTH) {
 		debug2("userauth_pubkey: SSH_BUG_PKAUTH");
@@ -131,6 +127,11 @@ userauth_pubkey(Authctxt *authctxt)
 		} else {
 			buffer_put_string(&b, session_id2, session_id2_len);
 		}
+		if (!authctxt->valid || authctxt->user == NULL) {
+			debug2("%s: disabled because of invalid user",
+			    __func__);
+			goto done;
+		}
 		/* reconstruct packet */
 		buffer_put_char(&b, SSH2_MSG_USERAUTH_REQUEST);
 		xasprintf(&userstyle, "%s%s%s", authctxt->user,
@@ -162,11 +163,14 @@ userauth_pubkey(Authctxt *authctxt)
 		    buffer_len(&b))) == 1)
 			authenticated = 1;
 		buffer_free(&b);
-		free(sig);
 	} else {
 		debug("test whether pkalg/pkblob are acceptable");
 		packet_check_eom();
-
+		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
@@ -192,6 +196,7 @@ done:
 		key_free(key);
 	free(pkalg);
 	free(pkblob);
+	free(sig);
 	return authenticated;
 }