diff -up openssh-8.7p1/kex.c.minsha1 openssh-8.7p1/kex.c

--- openssh-8.7p1/kex.c.minsha1	2021-12-20 17:38:51.438294309 +0100

+++ openssh-8.7p1/kex.c	2021-12-21 11:02:48.379991319 +0100

@@ -994,6 +994,35 @@ kex_choose_conf(struct ssh *ssh)

 		free(ext);

 	}

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

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

+		char *ext;

+

+		ext = match_list("rsa-sha2-256", peer[PROPOSAL_SERVER_HOST_KEY_ALGS], NULL);

+		if (ext) {

+			kex->flags |= KEX_RSA_SHA2_256_SUPPORTED;

+			free(ext);

+		}

+

+		ext = match_list("rsa-sha2-512", peer[PROPOSAL_SERVER_HOST_KEY_ALGS], NULL);

+		if (ext) {

+			kex->flags |= KEX_RSA_SHA2_512_SUPPORTED;

+			free(ext);

+		}

+

+		ext = match_list("rsa-sha2-256-cert-v01@openssh.com", peer[PROPOSAL_SERVER_HOST_KEY_ALGS], NULL);

+		if (ext) {

+			kex->flags |= KEX_RSA_SHA2_256_SUPPORTED;

+			free(ext);

+		}

+

+		ext = match_list("rsa-sha2-512-cert-v01@openssh.com", peer[PROPOSAL_SERVER_HOST_KEY_ALGS], NULL);

+		if (ext) {

+			kex->flags |= KEX_RSA_SHA2_512_SUPPORTED;

+			free(ext);

+		}

+	}

+

 	/* Algorithm Negotiation */

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

 	    sprop[PROPOSAL_KEX_ALGS])) != 0) {

diff -up openssh-8.7p1/kex.h.minsha1 openssh-8.7p1/kex.h

--- openssh-8.7p1/kex.h.minsha1	2021-12-20 17:38:51.430294240 +0100

+++ openssh-8.7p1/kex.h	2021-12-21 10:56:29.066735608 +0100

@@ -116,6 +116,8 @@ enum kex_exchange {

 #define KEX_INIT_SENT	0x0001

 #define KEX_INITIAL	0x0002

+#define KEX_RSA_SHA2_256_SUPPORTED 0x0004

+#define KEX_RSA_SHA2_512_SUPPORTED 0x0008

 struct sshenc {

 	char	*name;

diff -up openssh-8.7p1/serverloop.c.minsha1 openssh-8.7p1/serverloop.c

--- openssh-8.7p1/serverloop.c.minsha1	2021-08-20 06:03:49.000000000 +0200

+++ openssh-8.7p1/serverloop.c	2021-12-21 11:01:00.594047538 +0100

@@ -684,7 +685,7 @@ server_input_hostkeys_prove(struct ssh *

 	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;

 	u_char *sig = 0;

 	size_t blen, slen;

@@ -692,9 +693,11 @@ server_input_hostkeys_prove(struct ssh *

 	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));

 	while (ssh_packet_remaining(ssh) > 0) {

+		const char *pkexstr = NULL;

+		const char *rsa_sha2_256 = "rsa-sha2-256";

+		const char *rsa_sha2_512 = "rsa-sha2-512";

+

 		sshkey_free(key);

 		key = NULL;

 		if ((r = sshpkt_get_string_direct(ssh, &blob, &blen)) != 0 ||

@@ -726,8 +729,13 @@ server_input_hostkeys_prove(struct ssh *

 		 * 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;

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

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

+			pkexstr = rsa_sha2_512;

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

+			pkexstr = rsa_sha2_256;

+		}

+

 		if ((r = sshbuf_put_cstring(sigbuf,

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

 		    (r = sshbuf_put_stringb(sigbuf,

@@ -735,7 +743,7 @@ server_input_hostkeys_prove(struct ssh *

 		    (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 ||

+		    pkexstr)) != 0 ||

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

 			error_fr(r, "assemble signature");

 			goto out;