Backport of:

From 5d04ca6af739b82fd30d84d2783ca802ebfa1192 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Mon, 6 Jul 2026 07:53:30 +0000
Subject: [PATCH] upstream: Fix multiple RFC 4462 (GSSAPIAuthentication)
 compliance

problems

1) Remove an early failure return for GSSAPI authentication attempts
made for invalid accounts that yielded different behaviour for
valid vs invalid accounts.

2) Fix a situation where some GSSAPI requestes were not correctly
subjected to MaxAuthTries.

3) Fix a moderate pre-authentication resource DoS related to #2.

Add missing logging for error cases.

Report and fixes from Manfred Kaiser, milCERT AT

OpenBSD-Commit-ID: ca0acdd64eea435d6f89534538a9eb404a5629d3
---
 auth2-gss.c | 55 ++++++++++++++++++++++++-----------------------------
 1 file changed, 25 insertions(+), 30 deletions(-)

--- a/auth2-gss.c
+++ b/auth2-gss.c
@@ -146,12 +146,6 @@ userauth_gssapi(struct ssh *ssh, const c
 		return (0);
 	}
 
-	if (!authctxt->valid || authctxt->user == NULL) {
-		debug2_f("disabled because of invalid user");
-		free(doid);
-		return (0);
-	}
-
 	if (GSS_ERROR(PRIVSEP(ssh_gssapi_server_ctx(&ctxt, &goid)))) {
 		if (ctxt != NULL)
 			ssh_gssapi_delete_ctx(&ctxt);
@@ -213,8 +207,14 @@ input_gssapi_token(int type, u_int32_t p
 			    (r = sshpkt_send(ssh)) != 0)
 				fatal_fr(r, "send ERRTOK packet");
 		}
+		logit("Failed gssapi-with-mic for %s%.100s "
+		    "from %.200s port %d ssh2",
+		    authctxt->valid ? "" : "invalid user ",
+		    authctxt->user,
+		    ssh_remote_ipaddr(ssh), ssh_remote_port(ssh));
 		authctxt->postponed = 0;
 		ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
+		ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, NULL);
 		userauth_finish(ssh, 0, "gssapi-with-mic", NULL);
 	} else {
 		if (send_tok.length != 0) {
@@ -226,14 +226,18 @@ input_gssapi_token(int type, u_int32_t p
 				fatal_fr(r, "send TOKEN packet");
 		}
 		if (maj_status == GSS_S_COMPLETE) {
-			ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
-			if (flags & GSS_C_INTEG_FLAG)
-				ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_MIC,
+			ssh_dispatch_set(ssh,
+			    SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
+			/* note: keep ERRTOK handler as per RFC 4462 s3.4 */
+			if (flags & GSS_C_INTEG_FLAG) {
+				ssh_dispatch_set(ssh,
+				    SSH2_MSG_USERAUTH_GSSAPI_MIC,
 				    &input_gssapi_mic);
-			else
+			} else {
 				ssh_dispatch_set(ssh,
 				    SSH2_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE,
 				    &input_gssapi_exchange_complete);
+			}
 		}
 	}
 
@@ -245,10 +249,6 @@ static int
 input_gssapi_errtok(int type, u_int32_t plen, struct ssh *ssh)
 {
 	Authctxt *authctxt = ssh->authctxt;
-	Gssctxt *gssctxt;
-	gss_buffer_desc send_tok = GSS_C_EMPTY_BUFFER;
-	gss_buffer_desc recv_tok;
-	OM_uint32 maj_status;
 	int r;
 	u_char *p;
 	size_t len;
@@ -256,26 +256,20 @@ input_gssapi_errtok(int type, u_int32_t
 	if (authctxt == NULL || (authctxt->methoddata == NULL && !use_privsep))
 		fatal("No authentication or GSSAPI context");
 
-	gssctxt = authctxt->methoddata;
-	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0 ||
+	/* Minimal error handling - just cancel auth and return FAILURE */
+	if ((r = sshpkt_get_string_direct(ssh, NULL, NULL)) != 0 ||
 	    (r = sshpkt_get_end(ssh)) != 0)
 		fatal_fr(r, "parse packet");
-	recv_tok.value = p;
-	recv_tok.length = len;
-
-	/* Push the error token into GSSAPI to see what it says */
-	maj_status = PRIVSEP(ssh_gssapi_accept_ctx(gssctxt, &recv_tok,
-	    &send_tok, NULL));
-
-	free(recv_tok.value);
-
-	/* We can't return anything to the client, even if we wanted to */
+	logit("Failed gssapi-with-mic for %s%.100s from %.200s port %d ssh2",
+	    authctxt->valid ? "" : "invalid user ",
+	    authctxt->user,
+	    ssh_remote_ipaddr(ssh), ssh_remote_port(ssh));
+	authctxt->postponed = 0;
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, NULL);
-
-	/* The client will have already moved on to the next auth */
-
-	gss_release_buffer(&maj_status, &send_tok);
+	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_MIC, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE, NULL);
+	userauth_finish(ssh, 0, "gssapi-with-mic", NULL);
 	return 0;
 }
 
