diff --color -ru a/clientloop.c b/clientloop.c

--- a/clientloop.c	2022-06-29 16:35:06.677597259 +0200

+++ b/clientloop.c	2022-06-29 16:40:29.737926205 +0200

@@ -116,6 +116,9 @@

 #include "ssh-gss.h"

 #endif

+/* Permitted RSA signature algorithms for UpdateHostkeys proofs */

+#define HOSTKEY_PROOF_RSA_ALGS	"rsa-sha2-512,rsa-sha2-256"

+

 /* import options */

 extern Options options;

@@ -2110,8 +2113,10 @@

 	struct hostkeys_update_ctx *ctx = (struct hostkeys_update_ctx *)_ctx;

 	size_t i, ndone;

 	struct sshbuf *signdata;

-	int r, kexsigtype, use_kexsigtype;

+	int r, plaintype;

 	const u_char *sig;

+	const char *rsa_kexalg = NULL;

+	char *alg = NULL;

 	size_t siglen;

 	if (ctx->nnew == 0)

@@ -2122,9 +2127,9 @@

 		hostkeys_update_ctx_free(ctx);

 		return;

 	}

-	kexsigtype = sshkey_type_plain(

-	    sshkey_type_from_name(ssh->kex->hostkey_alg));

-

+	if (sshkey_type_plain(sshkey_type_from_name(

+	    ssh->kex->hostkey_alg)) == KEY_RSA)

+		rsa_kexalg = ssh->kex->hostkey_alg;

 	if ((signdata = sshbuf_new()) == NULL)

 		fatal_f("sshbuf_new failed");

 	/*

@@ -2135,6 +2140,7 @@

 	for (ndone = i = 0; i < ctx->nkeys; i++) {

 		if (ctx->keys_match[i])

 			continue;

+		plaintype = sshkey_type_plain(ctx->keys[i]->type);

 		/* Prepare data to be signed: session ID, unique string, key */

 		sshbuf_reset(signdata);

 		if ( (r = sshbuf_put_cstring(signdata,

@@ -2148,19 +2154,33 @@

 			error_fr(r, "parse sig");

 			goto out;

 		}

+		if ((r = sshkey_get_sigtype(sig, siglen, &alg)) != 0) {

+			error_fr(r, "server gave unintelligible signature "

+				"for %s key %zu", sshkey_type(ctx->keys[i]), i);

+			goto out;

+		}

 		/*

-		 * For RSA keys, prefer to use the signature type negotiated

-		 * during KEX to the default (SHA1).

+		 * Special case for RSA keys: if a RSA hostkey was negotiated,

+		 * then use its signature type for verification of RSA hostkey

+		 * proofs. Otherwise, accept only RSA-SHA256/512 signatures.

 		 */

-		use_kexsigtype = kexsigtype == KEY_RSA &&

-		    sshkey_type_plain(ctx->keys[i]->type) == KEY_RSA;

-		debug3_f("verify %s key %zu using %s sigalg",

-		    sshkey_type(ctx->keys[i]), i,

-		    use_kexsigtype ? ssh->kex->hostkey_alg : "default");

+		if (plaintype == KEY_RSA && rsa_kexalg == NULL &&

+		    match_pattern_list(alg, HOSTKEY_PROOF_RSA_ALGS, 0) != 1) {

+			debug_f("server used untrusted RSA signature algorithm "

+				"%s for key %zu, disregarding", alg, i);

+			free(alg);

+			/* zap the key from the list */

+			sshkey_free(ctx->keys[i]);

+			ctx->keys[i] = NULL;

+			ndone++;

+			continue;

+		}

+		debug3_f("verify %s key %zu using sigalg %s",

+			sshkey_type(ctx->keys[i]), i, alg);

+		free(alg);

 		if ((r = sshkey_verify(ctx->keys[i], sig, siglen,

 		    sshbuf_ptr(signdata), sshbuf_len(signdata),

-		    use_kexsigtype ? ssh->kex->hostkey_alg : NULL, 0,

-		    NULL)) != 0) {

+		    plaintype == KEY_RSA ? rsa_kexalg : NULL, 0, NULL)) != 0) {

 			error_fr(r, "server gave bad signature for %s key %zu",

 			    sshkey_type(ctx->keys[i]), i);

 			goto out;

diff --color -ru a/kex.c b/kex.c

--- a/kex.c	2022-06-29 16:35:06.775599179 +0200

+++ b/kex.c	2022-06-29 16:42:00.839710940 +0200

@@ -959,6 +959,18 @@

 	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;

+}

+

 static int

 kex_choose_conf(struct ssh *ssh)

 {

@@ -994,6 +1006,16 @@

 		free(ext);

 	}

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

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

+		if (has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],

+		    "rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com"))

+			kex->flags |= KEX_RSA_SHA2_256_SUPPORTED;

+		if (has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],

+		    "rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com"))

+			kex->flags |= KEX_RSA_SHA2_512_SUPPORTED;

+	}

+

 	/* Algorithm Negotiation */

 	if ((r = choose_kex(kex, cprop[PROPOSAL_KEX_ALGS],

 	    sprop[PROPOSAL_KEX_ALGS])) != 0) {

diff --color -ru a/kex.h b/kex.h

--- a/kex.h	2022-06-29 16:35:06.766599003 +0200

+++ b/kex.h	2022-06-29 16:42:24.199168567 +0200

@@ -116,6 +116,8 @@

 #define KEX_INIT_SENT	0x0001

 #define KEX_INITIAL	0x0002

+#define KEX_RSA_SHA2_256_SUPPORTED      0x0008 /* only set in server for now */

+#define KEX_RSA_SHA2_512_SUPPORTED      0x0010 /* only set in server for now */

 struct sshenc {

 	char	*name;

diff --color -ru a/serverloop.c b/serverloop.c

--- a/serverloop.c	2021-08-20 06:03:49.000000000 +0200

+++ b/serverloop.c	2022-06-29 16:45:05.902336428 +0200

@@ -684,16 +684,18 @@

 	struct sshbuf *resp = NULL;

 	struct sshbuf *sigbuf = NULL;

 	struct sshkey *key = NULL, *key_pub = NULL, *key_prv = NULL;

-	int r, ndx, kexsigtype, use_kexsigtype, success = 0;

+	int r, ndx, success = 0;

 	const u_char *blob;

+	const char *sigalg, *kex_rsa_sigalg = NULL;

 	u_char *sig = 0;

 	size_t blen, slen;

 	if ((resp = sshbuf_new()) == NULL || (sigbuf = sshbuf_new()) == NULL)

 		fatal_f("sshbuf_new");

-	kexsigtype = sshkey_type_plain(

-	    sshkey_type_from_name(ssh->kex->hostkey_alg));

+	if (sshkey_type_plain(sshkey_type_from_name(

+	    ssh->kex->hostkey_alg)) == KEY_RSA)

+		kex_rsa_sigalg = ssh->kex->hostkey_alg;

 	while (ssh_packet_remaining(ssh) > 0) {

 		sshkey_free(key);

 		key = NULL;

@@ -726,16 +728,24 @@

 		 * For RSA keys, prefer to use the signature type negotiated

 		 * during KEX to the default (SHA1).

 		 */

-		use_kexsigtype = kexsigtype == KEY_RSA &&

-		    sshkey_type_plain(key->type) == KEY_RSA;

+		sigalg = NULL;

+		if (sshkey_type_plain(key->type) == KEY_RSA) {

+			if (kex_rsa_sigalg != NULL)

+				sigalg = kex_rsa_sigalg;

+			else if (ssh->kex->flags & KEX_RSA_SHA2_512_SUPPORTED)

+				sigalg = "rsa-sha2-512";

+			else if (ssh->kex->flags & KEX_RSA_SHA2_256_SUPPORTED)

+				sigalg = "rsa-sha2-256";

+		}

+		debug3_f("sign %s key (index %d) using sigalg %s",

+		sshkey_type(key), ndx, sigalg == NULL ? "default" : sigalg);

 		if ((r = sshbuf_put_cstring(sigbuf,

 		    "hostkeys-prove-00@openssh.com")) != 0 ||

 		    (r = sshbuf_put_stringb(sigbuf,

 		    ssh->kex->session_id)) != 0 ||

 		    (r = sshkey_puts(key, sigbuf)) != 0 ||

 		    (r = ssh->kex->sign(ssh, key_prv, key_pub, &sig, &slen,

-		    sshbuf_ptr(sigbuf), sshbuf_len(sigbuf),

-		    use_kexsigtype ? ssh->kex->hostkey_alg : NULL)) != 0 ||

+		    sshbuf_ptr(sigbuf), sshbuf_len(sigbuf), sigalg)) != 0 ||

 		    (r = sshbuf_put_string(resp, sig, slen)) != 0) {

 			error_fr(r, "assemble signature");

 			goto out;