diff --git a/PROTOCOL b/PROTOCOL

index d453c779..ded935eb 100644

--- a/PROTOCOL

+++ b/PROTOCOL

@@ -137,6 +137,32 @@ than as a named global or channel request to allow pings with very

 described at:

 http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519

+1.9 transport: strict key exchange extension

+

+OpenSSH supports a number of transport-layer hardening measures under

+a "strict KEX" feature. This feature is signalled similarly to the

+RFC8308 ext-info feature: by including a additional algorithm in the

+initiial SSH2_MSG_KEXINIT kex_algorithms field. The client may append

+"kex-strict-c-v00@openssh.com" to its kex_algorithms and the server

+may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms

+are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored

+if they are present in subsequent SSH2_MSG_KEXINIT packets.

+

+When an endpoint that supports this extension observes this algorithm

+name in a peer's KEXINIT packet, it MUST make the following changes to

+the the protocol:

+

+a) During initial KEX, terminate the connection if any unexpected or

+   out-of-sequence packet is received. This includes terminating the

+   connection if the first packet received is not SSH2_MSG_KEXINIT.

+   Unexpected packets for the purpose of strict KEX include messages

+   that are otherwise valid at any time during the connection such as

+   SSH2_MSG_DEBUG and SSH2_MSG_IGNORE.

+b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the

+   packet sequence number to zero. This behaviour persists for the

+   duration of the connection (i.e. not just the first

+   SSH2_MSG_NEWKEYS).

+

 2. Connection protocol changes

 2.1. connection: Channel write close extension "eow@openssh.com"

diff --git a/kex.c b/kex.c

index aa5e792d..d478ff6e 100644

--- a/kex.c

+++ b/kex.c

@@ -65,7 +65,7 @@

 #endif

 /* prototype */

-static int kex_choose_conf(struct ssh *);

+static int kex_choose_conf(struct ssh *, uint32_t seq);

 static int kex_input_newkeys(int, u_int32_t, struct ssh *);

 static const char *proposal_names[PROPOSAL_MAX] = {

@@ -177,6 +177,18 @@ kex_names_valid(const char *names)

 	return 1;

 }

+/* returns non-zero if proposal contains any algorithm from algs */

+static int

+has_any_alg(const char *proposal, const char *algs)

+{

+	char *cp;

+

+	if ((cp = match_list(proposal, algs, NULL)) == NULL)

+		return 0;

+	free(cp);

+	return 1;

+}

+

 /*

  * Concatenate algorithm names, avoiding duplicates in the process.

  * Caller must free returned string.

@@ -184,7 +196,7 @@ kex_names_valid(const char *names)

 char *

 kex_names_cat(const char *a, const char *b)

 {

-	char *ret = NULL, *tmp = NULL, *cp, *p, *m;

+	char *ret = NULL, *tmp = NULL, *cp, *p;

 	size_t len;

 	if (a == NULL || *a == '\0')

@@ -201,10 +213,8 @@ kex_names_cat(const char *a, const char *b)

 	}

 	strlcpy(ret, a, len);

 	for ((p = strsep(&cp, ",")); p && *p != '\0'; (p = strsep(&cp, ","))) {

-		if ((m = match_list(ret, p, NULL)) != NULL) {

-			free(m);

+		if (has_any_alg(ret, p))

 			continue; /* Algorithm already present */

-		}

 		if (strlcat(ret, ",", len) >= len ||

 		    strlcat(ret, p, len) >= len) {

 			free(tmp);

@@ -466,7 +485,12 @@ kex_protocol_error(int type, u_int32_t seq, struct ssh *ssh)

 {

 	int r;

-	error("kex protocol error: type %d seq %u", type, seq);

+	/* If in strict mode, any unexpected message is an error */

+	if ((ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict) {

+		ssh_packet_disconnect(ssh, "strict KEX violation: "

+		    "unexpected packet type %u (seqnr %u)", type, seq);

+	}

+	error_f("type %u seq %u", type, seq);

 	if ((r = sshpkt_start(ssh, SSH2_MSG_UNIMPLEMENTED)) != 0 ||

 	    (r = sshpkt_put_u32(ssh, seq)) != 0 ||

 	    (r = sshpkt_send(ssh)) != 0)

@@ -548,6 +572,11 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)

 	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &kex_protocol_error);

 	if ((r = sshpkt_get_u32(ssh, &ninfo)) != 0)

 		return r;

+ 	if (ninfo >= 1024) {

+ 		error("SSH2_MSG_EXT_INFO with too many entries, expected "

+ 		    "<=1024, received %u", ninfo);

+		return dispatch_protocol_error(type, seq, ssh);

+ 	}

 	for (i = 0; i < ninfo; i++) {

 		if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)

 			return r;

@@ -681,7 +705,7 @@ kex_input_kexinit(int type, u_int32_t seq, struct ssh *ssh)

 		error_f("no kex");

 		return SSH_ERR_INTERNAL_ERROR;

 	}

-	ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, NULL);

+	ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_protocol_error);

 	ptr = sshpkt_ptr(ssh, &dlen);

 	if ((r = sshbuf_put(kex->peer, ptr, dlen)) != 0)

 		return r;

@@ -717,7 +741,7 @@ kex_input_kexinit(int type, u_int32_t seq, struct ssh *ssh)

 	if (!(kex->flags & KEX_INIT_SENT))

 		if ((r = kex_send_kexinit(ssh)) != 0)

 			return r;

-	if ((r = kex_choose_conf(ssh)) != 0)

+	if ((r = kex_choose_conf(ssh, seq)) != 0)

 		return r;

 	if (kex->kex_type < KEX_MAX && kex->kex[kex->kex_type] != NULL)

@@ -981,20 +1005,14 @@ proposals_match(char *my[PROPOSAL_MAX], char *peer[PROPOSAL_MAX])

 	return (1);

 }

-/* returns non-zero if proposal contains any algorithm from algs */

 static int

-has_any_alg(const char *proposal, const char *algs)

+kexalgs_contains(char **peer, const char *ext)

 {

-	char *cp;

-

-	if ((cp = match_list(proposal, algs, NULL)) == NULL)

-		return 0;

-	free(cp);

-	return 1;

+	return has_any_alg(peer[PROPOSAL_KEX_ALGS], ext);

 }

 static int

-kex_choose_conf(struct ssh *ssh)

+kex_choose_conf(struct ssh *ssh, uint32_t seq)

 {

 	struct kex *kex = ssh->kex;

 	struct newkeys *newkeys;

@@ -1019,13 +1037,23 @@ kex_choose_conf(struct ssh *ssh)

 		sprop=peer;

 	}

-	/* Check whether client supports ext_info_c */

-	if (kex->server && (kex->flags & KEX_INITIAL)) {

-		char *ext;

-

-		ext = match_list("ext-info-c", peer[PROPOSAL_KEX_ALGS], NULL);

-		kex->ext_info_c = (ext != NULL);

-		free(ext);

+	/* Check whether peer supports ext_info/kex_strict */

+	if ((kex->flags & KEX_INITIAL) != 0) {

+		if (kex->server) {

+			kex->ext_info_c = kexalgs_contains(peer, "ext-info-c");

+			kex->kex_strict = kexalgs_contains(peer,

+			    "kex-strict-c-v00@openssh.com");

+		} else {

+			kex->kex_strict = kexalgs_contains(peer,

+			    "kex-strict-s-v00@openssh.com");

+		}

+		if (kex->kex_strict) {

+			debug3_f("will use strict KEX ordering");

+			if (seq != 0)

+				ssh_packet_disconnect(ssh,

+				    "strict KEX violation: "

+				    "KEXINIT was not the first packet");

+		}

 	}

 	/* Check whether client supports rsa-sha2 algorithms */

diff --git a/kex.h b/kex.h

index 5f7ef784..272ebb43 100644

--- a/kex.h

+++ b/kex.h

@@ -149,6 +149,7 @@ struct kex {

 	u_int	kex_type;

 	char	*server_sig_algs;

 	int	ext_info_c;

+	int	kex_strict;

 	struct sshbuf *my;

 	struct sshbuf *peer;

 	struct sshbuf *client_version;

diff --git a/packet.c b/packet.c

index 52017def..beb214f9 100644

--- a/packet.c

+++ b/packet.c

@@ -1207,8 +1207,13 @@ ssh_packet_send2_wrapped(struct ssh *ssh)

 	sshbuf_dump(state->output, stderr);

 #endif

 	/* increment sequence number for outgoing packets */

-	if (++state->p_send.seqnr == 0)

+	if (++state->p_send.seqnr == 0) {

+		if ((ssh->kex->flags & KEX_INITIAL) != 0) {

+			ssh_packet_disconnect(ssh, "outgoing sequence number "

+			    "wrapped during initial key exchange");

+		}

 		logit("outgoing seqnr wraps around");

+	}

 	if (++state->p_send.packets == 0)

 		if (!(ssh->compat & SSH_BUG_NOREKEY))

 			return SSH_ERR_NEED_REKEY;

@@ -1216,6 +1221,11 @@ ssh_packet_send2_wrapped(struct ssh *ssh)

 	state->p_send.bytes += len;

 	sshbuf_reset(state->outgoing_packet);

+	if (type == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {

+		debug_f("resetting send seqnr %u", state->p_send.seqnr);

+		state->p_send.seqnr = 0;

+	}

+

 	if (type == SSH2_MSG_NEWKEYS)

 		r = ssh_set_newkeys(ssh, MODE_OUT);

 	else if (type == SSH2_MSG_USERAUTH_SUCCESS && state->server_side)

@@ -1344,8 +1354,7 @@ ssh_packet_read_seqnr(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)

 	/* Stay in the loop until we have received a complete packet. */

 	for (;;) {

 		/* Try to read a packet from the buffer. */

-		r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p);

-		if (r != 0)

+		if ((r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p)) != 0)

 			break;

 		/* If we got a packet, return it. */

 		if (*typep != SSH_MSG_NONE)

@@ -1629,10 +1615,16 @@ ssh_packet_read_poll2(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)

 		if ((r = sshbuf_consume(state->input, mac->mac_len)) != 0)

 			goto out;

 	}

+

 	if (seqnr_p != NULL)

 		*seqnr_p = state->p_read.seqnr;

-	if (++state->p_read.seqnr == 0)

+	if (++state->p_read.seqnr == 0) {

+		if ((ssh->kex->flags & KEX_INITIAL) != 0) {

+			ssh_packet_disconnect(ssh, "incoming sequence number "

+			    "wrapped during initial key exchange");

+		}

 		logit("incoming seqnr wraps around");

+	}

 	if (++state->p_read.packets == 0)

 		if (!(ssh->compat & SSH_BUG_NOREKEY))

 			return SSH_ERR_NEED_REKEY;

@@ -1698,6 +1690,10 @@ ssh_packet_read_poll2(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)

 #endif

 	/* reset for next packet */

 	state->packlen = 0;

+	if (*typep == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {

+		debug_f("resetting read seqnr %u", state->p_read.seqnr);

+		state->p_read.seqnr = 0;

+	}

 	if ((r = ssh_packet_check_rekey(ssh)) != 0)

 		return r;

@@ -1720,10 +1716,39 @@ ssh_packet_read_poll_seqnr(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)

 		r = ssh_packet_read_poll2(ssh, typep, seqnr_p);

 		if (r != 0)

 			return r;

-		if (*typep) {

-			state->keep_alive_timeouts = 0;

-			DBG(debug("received packet type %d", *typep));

+		if (*typep == 0) {

+			/* no message ready */

+			return 0;

 		}

+		state->keep_alive_timeouts = 0;

+		DBG(debug("received packet type %d", *typep));

+

+		/* Always process disconnect messages */

+		if (*typep == SSH2_MSG_DISCONNECT) {

+			if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||

+			    (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)

+				return r;

+			/* Ignore normal client exit notifications */

+			do_log2(ssh->state->server_side &&

+			    reason == SSH2_DISCONNECT_BY_APPLICATION ?

+			    SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,

+			    "Received disconnect from %s port %d:"

+			    "%u: %.400s", ssh_remote_ipaddr(ssh),

+			    ssh_remote_port(ssh), reason, msg);

+			free(msg);

+			return SSH_ERR_DISCONNECTED;

+		}

+

+		/*

+		 * Do not implicitly handle any messages here during initial

+		 * KEX when in strict mode. They will be need to be allowed

+		 * explicitly by the KEX dispatch table or they will generate

+		 * protocol errors.

+		 */

+		if (ssh->kex != NULL &&

+		    (ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict)

+			return 0;

+		/* Implicitly handle transport-level messages */

 		switch (*typep) {

 		case SSH2_MSG_IGNORE:

 			debug3("Received SSH2_MSG_IGNORE");

@@ -1738,19 +1763,6 @@ ssh_packet_read_poll_seqnr(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)

 			debug("Remote: %.900s", msg);

 			free(msg);

 			break;

-		case SSH2_MSG_DISCONNECT:

-			if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||

-			    (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)

-				return r;

-			/* Ignore normal client exit notifications */

-			do_log2(ssh->state->server_side &&

-			    reason == SSH2_DISCONNECT_BY_APPLICATION ?

-			    SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,

-			    "Received disconnect from %s port %d:"

-			    "%u: %.400s", ssh_remote_ipaddr(ssh),

-			    ssh_remote_port(ssh), reason, msg);

-			free(msg);

-			return SSH_ERR_DISCONNECTED;

 		case SSH2_MSG_UNIMPLEMENTED:

 			if ((r = sshpkt_get_u32(ssh, &seqnr)) != 0)

 				return r;

@@ -2242,6 +2254,7 @@ kex_to_blob(struct sshbuf *m, struct kex *kex)

 	    (r = sshbuf_put_u32(m, kex->hostkey_type)) != 0 ||

 	    (r = sshbuf_put_u32(m, kex->hostkey_nid)) != 0 ||

 	    (r = sshbuf_put_u32(m, kex->kex_type)) != 0 ||

+	    (r = sshbuf_put_u32(m, kex->kex_strict)) != 0 ||

 	    (r = sshbuf_put_stringb(m, kex->my)) != 0 ||

 	    (r = sshbuf_put_stringb(m, kex->peer)) != 0 ||

 	    (r = sshbuf_put_stringb(m, kex->client_version)) != 0 ||

@@ -2404,6 +2417,7 @@ kex_from_blob(struct sshbuf *m, struct kex **kexp)

 	    (r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_type)) != 0 ||

 	    (r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_nid)) != 0 ||

 	    (r = sshbuf_get_u32(m, &kex->kex_type)) != 0 ||

+	    (r = sshbuf_get_u32(m, &kex->kex_strict)) != 0 ||

 	    (r = sshbuf_get_stringb(m, kex->my)) != 0 ||

 	    (r = sshbuf_get_stringb(m, kex->peer)) != 0 ||

 	    (r = sshbuf_get_stringb(m, kex->client_version)) != 0 ||

@@ -2732,6 +2746,7 @@ sshpkt_disconnect(struct ssh *ssh, const char *fmt,...)

 	vsnprintf(buf, sizeof(buf), fmt, args);

 	va_end(args);

+	debug2_f("sending SSH2_MSG_DISCONNECT: %s", buf);

 	if ((r = sshpkt_start(ssh, SSH2_MSG_DISCONNECT)) != 0 ||

 	    (r = sshpkt_put_u32(ssh, SSH2_DISCONNECT_PROTOCOL_ERROR)) != 0 ||

 	    (r = sshpkt_put_cstring(ssh, buf)) != 0 ||

diff --git a/sshconnect2.c b/sshconnect2.c

index df6caf81..0cccbcc4 100644

--- a/sshconnect2.c

+++ b/sshconnect2.c

@@ -253,7 +253,8 @@ ssh_kex2(struct ssh *ssh, char *host, st

 		fatal_fr(r, "kex_assemble_namelist");

 	free(all_key);

-	if ((s = kex_names_cat(options.kex_algorithms, "ext-info-c")) == NULL)

+	if ((s = kex_names_cat(options.kex_algorithms,

+	    "ext-info-c,kex-strict-c-v00@openssh.com")) == NULL)

 		fatal_f("kex_names_cat");

 	myproposal[PROPOSAL_KEX_ALGS] = prop_kex = compat_kex_proposal(ssh, s);

 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =

@@ -358,7 +358,6 @@ struct cauthmethod {

 };

 static int input_userauth_service_accept(int, u_int32_t, struct ssh *);

-static int input_userauth_ext_info(int, u_int32_t, struct ssh *);

 static int input_userauth_success(int, u_int32_t, struct ssh *);

 static int input_userauth_failure(int, u_int32_t, struct ssh *);

 static int input_userauth_banner(int, u_int32_t, struct ssh *);

@@ -472,7 +471,7 @@ ssh_userauth2(struct ssh *ssh, const char *local_user,

 	ssh->authctxt = &authctxt;

 	ssh_dispatch_init(ssh, &input_userauth_error);

-	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &input_userauth_ext_info);

+	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, kex_input_ext_info);

 	ssh_dispatch_set(ssh, SSH2_MSG_SERVICE_ACCEPT, &input_userauth_service_accept);

 	ssh_dispatch_run_fatal(ssh, DISPATCH_BLOCK, &authctxt.success);	/* loop until success */

 	pubkey_cleanup(ssh);

@@ -531,12 +530,6 @@ input_userauth_service_accept(int type, u_int32_t seq, struct ssh *ssh)

 }

 /* ARGSUSED */

-static int

-input_userauth_ext_info(int type, u_int32_t seqnr, struct ssh *ssh)

-{

-	return kex_input_ext_info(type, seqnr, ssh);

-}

-

 void

 userauth(struct ssh *ssh, char *authlist)

 {

@@ -615,6 +608,7 @@ input_userauth_success(int type, u_int32_t seq, struct ssh *ssh)

 	free(authctxt->methoddata);

 	authctxt->methoddata = NULL;

 	authctxt->success = 1;			/* break out */

+	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, dispatch_protocol_error);

 	return 0;

 }

diff -up openssh-8.7p1/sshd.c.kexstrict openssh-8.7p1/sshd.c

--- openssh-8.7p1/sshd.c.kexstrict	2023-11-27 13:19:18.855433602 +0100

+++ openssh-8.7p1/sshd.c	2023-11-27 13:28:10.441325314 +0100

@@ -2531,10 +2531,14 @@ do_ssh2_kex(struct ssh *ssh)

 	struct kex *kex;

 	char *hostkey_types = NULL;

 	char *prop_kex = NULL, *prop_enc = NULL, *prop_hostkey = NULL;

+	char *cp;

 	int r;

-	myproposal[PROPOSAL_KEX_ALGS] = prop_kex = compat_kex_proposal(ssh,

-	    options.kex_algorithms);

+	if ((cp = kex_names_cat(options.kex_algorithms, 

+	   "kex-strict-s-v00@openssh.com")) == NULL)

+		fatal_f("kex_names_cat");

+

+	myproposal[PROPOSAL_KEX_ALGS] = prop_kex = compat_kex_proposal(ssh, cp);

 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =

 	    myproposal[PROPOSAL_ENC_ALGS_STOC] = prop_enc =

 	    compat_cipher_proposal(ssh, options.ciphers);

@@ -2586,7 +2586,7 @@ do_ssh2_kex(struct ssh *ssh)

 	if (gss && orig)

 		xasprintf(&newstr, "%s,%s", gss, orig);

 	else if (gss)

-		newstr = gss;

+		xasprintf(&newstr, "%s,%s", gss, "kex-strict-s-v00@openssh.com");

 	else if (orig)

 		newstr = orig;

@@ -2650,6 +2654,7 @@ do_ssh2_kex(struct ssh *ssh)

 #endif

 	free(prop_kex);

 	free(prop_enc);

+	free(cp);

 	free(prop_hostkey);

 	debug("KEX done");

 }