From 83eeaca768f50d69a5de9ed100074418929976a5 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Fri, 30 Oct 2026 08:39:48 +0000
Subject: [PATCH] upstream: when StrictHostkeyChecking=no and an
 changed/revoked

host key was permitted by the user anyway, ssh(1) turns off a bunch of more
risk options including various forwardings. Unfortunately we had previously
neglected to turn them off over the mux channel.

Anthropic ANT-2026-VFW3JCKW

OpenBSD-Commit-ID: ad9c14298107c0d8891f68baddef63bfc976cac9
---
 clientloop.c | 10 +++++++++-
 mux.c        | 14 +++++++++++++-
 sshconnect.c | 11 ++++++++++-
 3 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/clientloop.c b/clientloop.c
index 0df129113..6e6526aee 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.c,v 1.427 2026/09/16 23:26:41 jsg Exp $ */
+/* $OpenBSD: clientloop.c,v 1.428 2026/10/30 08:39:48 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -128,6 +128,9 @@ extern char *host;
  */
 extern char *forward_agent_sock_path;
 
+/* Set if we continued past a host key verification failure (sshconnect.c) */
+extern int hostkey_untrusted;
+
 /*
  * Flag to indicate that we have received a window change signal which has
  * not yet been processed.  This will cause a message indicating the new
@@ -1068,6 +1071,11 @@ process_cmdline(struct ssh *ssh)
 		}
 		logit("Canceled forwarding.");
 	} else {
+		if (hostkey_untrusted) {
+			logit("Port forwarding is disabled to avoid "
+			    "monster-in-the-middle attacks.");
+			goto out;
+		}
 		/* -R specs can be both dynamic or not, so check both. */
 		if (remote) {
 			if (!parse_forward(&fwd, s, 0, remote) &&
diff --git a/mux.c b/mux.c
index 5b34dba2b..d06fadefe 100644
--- a/mux.c
+++ b/mux.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: mux.c,v 1.118 2026/10/30 08:27:49 djm Exp $ */
+/* $OpenBSD: mux.c,v 1.119 2026/10/30 08:39:48 djm Exp $ */
 /*
  * Copyright (c) 2002-2008 Damien Miller <djm@openbsd.org>
  *
@@ -58,6 +58,9 @@ extern char *host;
 extern struct sshbuf *command;
 extern volatile sig_atomic_t quit_pending;
 
+/* from sshconnect.c */
+extern int hostkey_untrusted;
+
 /* Context for session open confirmation callback */
 struct mux_session_confirm_ctx {
 	u_int want_tty;
@@ -837,6 +840,15 @@ mux_master_process_open_fwd(struct ssh *ssh, u_int rid,
 		break;
 	}
 
+	/* Forwarding was disabled when the host key failed verification */
+	if (hostkey_untrusted) {
+		logit_f("refusing %s: host key is not trusted", fwd_desc);
+		reply_error(reply, MUX_S_PERMISSION_DENIED, rid,
+		    "Port forwarding is disabled to avoid "
+		    "man-in-the-middle attacks");
+		goto out;
+	}
+
 	if (options.control_master == SSHCTL_MASTER_ASK ||
 	    options.control_master == SSHCTL_MASTER_AUTO_ASK) {
 		if (!ask_permission("Open %s on %s?", fwd_desc, host)) {
diff --git a/sshconnect.c b/sshconnect.c
index 202820be3..f3aa89da5 100644
--- a/sshconnect.c
+++ b/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.392 2026/10/30 08:21:42 djm Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.393 2026/10/30 08:39:48 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -58,6 +58,14 @@
 
 struct sshkey *previous_host_key = NULL;
 
+/*
+ * Set if the host key failed verification and we continued anyway
+ * (StrictHostKeyChecking=no). Used to refuse MITM-able forwarding
+ * requests that arrive after the connection is established, e.g.
+ * from multiplexing clients.
+ */
+int hostkey_untrusted = 0;
+
 static int matching_host_key_dns = 0;
 
 static volatile pid_t proxy_command_pid = 0;
@@ -1417,6 +1425,7 @@ check_host_key(char *hostname, const struct ssh_conn_info *cinfo,
 		 * the connection but without MITM-able authentication or
 		 * forwarding.
 		 */
+		hostkey_untrusted = 1;
 		if (options.password_authentication) {
 			error("Password authentication is disabled to avoid "
 			    "man-in-the-middle attacks.");