diff -up openssh-8.7p1/compat.c.sshrsacheck openssh-8.7p1/compat.c

--- openssh-8.7p1/compat.c.sshrsacheck	2023-01-12 13:29:06.338710923 +0100

+++ openssh-8.7p1/compat.c	2023-01-12 13:29:06.357711165 +0100

@@ -43,6 +43,7 @@ void

 compat_banner(struct ssh *ssh, const char *version)

 {

 	int i;

+	int forbid_ssh_rsa = 0;

 	static struct {

 		char	*pat;

 		int	bugs;

@@ -145,16 +146,21 @@ compat_banner(struct ssh *ssh, const cha

 	};

 	/* process table, return first match */

+	forbid_ssh_rsa = (ssh->compat & SSH_RH_RSASIGSHA);

 	ssh->compat = 0;

 	for (i = 0; check[i].pat; i++) {

 		if (match_pattern_list(version, check[i].pat, 0) == 1) {

 			debug_f("match: %s pat %s compat 0x%08x",

 			    version, check[i].pat, check[i].bugs);

 			ssh->compat = check[i].bugs;

+	if (forbid_ssh_rsa)

+		ssh->compat |= SSH_RH_RSASIGSHA;

 			return;

 		}

 	}

 	debug_f("no match: %s", version);

+	if (forbid_ssh_rsa)

+		ssh->compat |= SSH_RH_RSASIGSHA;

 }

 /* Always returns pointer to allocated memory, caller must free. */

diff -up openssh-8.7p1/compat.h.sshrsacheck openssh-8.7p1/compat.h

--- openssh-8.7p1/compat.h.sshrsacheck	2021-08-20 06:03:49.000000000 +0200

+++ openssh-8.7p1/compat.h	2023-01-12 13:29:06.358711178 +0100

@@ -30,7 +30,7 @@

 #define SSH_BUG_UTF8TTYMODE	0x00000001

 #define SSH_BUG_SIGTYPE		0x00000002

 #define SSH_BUG_SIGTYPE74	0x00000004

-/* #define unused		0x00000008 */

+#define SSH_RH_RSASIGSHA	0x00000008

 #define SSH_OLD_SESSIONID	0x00000010

 /* #define unused		0x00000020 */

 #define SSH_BUG_DEBUG		0x00000040

diff -up openssh-8.7p1/monitor.c.sshrsacheck openssh-8.7p1/monitor.c

--- openssh-8.7p1/monitor.c.sshrsacheck	2023-01-20 13:07:54.279676981 +0100

+++ openssh-8.7p1/monitor.c	2023-01-20 15:01:07.007821379 +0100

@@ -660,11 +660,12 @@ mm_answer_sign(struct ssh *ssh, int sock

 	struct sshkey *key;

 	struct sshbuf *sigbuf = NULL;

 	u_char *p = NULL, *signature = NULL;

-	char *alg = NULL;

+	char *alg = NULL, *effective_alg;

 	size_t datlen, siglen, alglen;

 	int r, is_proof = 0;

 	u_int keyid, compat;

 	const char proof_req[] = "hostkeys-prove-00@openssh.com";

+	const char safe_rsa[]  = "rsa-sha2-256";

 	debug3_f("entering");

@@ -719,18 +720,30 @@ mm_answer_sign(struct ssh *ssh, int sock

 	}

 	if ((key = get_hostkey_by_index(keyid)) != NULL) {

-		if ((r = sshkey_sign(key, &signature, &siglen, p, datlen, alg,

+		if (ssh->compat & SSH_RH_RSASIGSHA && strcmp(alg, "ssh-rsa") == 0

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

+			effective_alg = safe_rsa;

+		} else {

+			effective_alg = alg;

+		}

+		if ((r = sshkey_sign(key, &signature, &siglen, p, datlen, effective_alg,

 		    options.sk_provider, NULL, compat)) != 0)

 			fatal_fr(r, "sign");

 	} else if ((key = get_hostkey_public_by_index(keyid, ssh)) != NULL &&

 	    auth_sock > 0) {

+		if (ssh->compat & SSH_RH_RSASIGSHA && strcmp(alg, "ssh-rsa") == 0

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

+			effective_alg = safe_rsa;

+		} else {

+			effective_alg = alg;

+		}

 		if ((r = ssh_agent_sign(auth_sock, key, &signature, &siglen,

-		    p, datlen, alg, compat)) != 0)

+		    p, datlen, effective_alg, compat)) != 0)

 			fatal_fr(r, "agent sign");

 	} else

 		fatal_f("no hostkey from index %d", keyid);

-	debug3_f("%s %s signature len=%zu", alg,

+	debug3_f("%s (effective: %s) %s signature len=%zu", alg, effective_alg,

 	    is_proof ? "hostkey proof" : "KEX", siglen);

 	sshbuf_reset(m);

diff -up openssh-8.7p1/regress/unittests/kex/test_kex.c.sshrsacheck openssh-8.7p1/regress/unittests/kex/test_kex.c

--- openssh-8.7p1/regress/unittests/kex/test_kex.c.sshrsacheck	2023-01-26 13:34:52.645743677 +0100

+++ openssh-8.7p1/regress/unittests/kex/test_kex.c	2023-01-26 13:36:56.220745823 +0100

@@ -97,7 +97,8 @@ do_kex_with_key(char *kex, int keytype,

 	memcpy(kex_params.proposal, myproposal, sizeof(myproposal));

 	if (kex != NULL)

 		kex_params.proposal[PROPOSAL_KEX_ALGS] = kex;

-	keyname = strdup(sshkey_ssh_name(private));

+	keyname = (strcmp(sshkey_ssh_name(private), "ssh-rsa")) ?

+		strdup(sshkey_ssh_name(private)) : strdup("rsa-sha2-256");

 	ASSERT_PTR_NE(keyname, NULL);

 	kex_params.proposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = keyname;

 	ASSERT_INT_EQ(ssh_init(&client, 0, &kex_params), 0);

diff -up openssh-8.7p1/regress/unittests/sshkey/test_file.c.sshrsacheck openssh-8.7p1/regress/unittests/sshkey/test_file.c

--- openssh-8.7p1/regress/unittests/sshkey/test_file.c.sshrsacheck	2023-01-26 12:04:55.946343408 +0100

+++ openssh-8.7p1/regress/unittests/sshkey/test_file.c	2023-01-26 12:06:35.235164432 +0100

@@ -110,6 +110,7 @@ sshkey_file_tests(void)

 	sshkey_free(k2);

 	TEST_DONE();

+	/* Skip this test, SHA1 signatures are not supported

 	TEST_START("load RSA cert with SHA1 signature");

 	ASSERT_INT_EQ(sshkey_load_cert(test_data_file("rsa_1_sha1"), &k2), 0);

 	ASSERT_PTR_NE(k2, NULL);

@@ -117,7 +118,7 @@ sshkey_file_tests(void)

 	ASSERT_INT_EQ(sshkey_equal_public(k1, k2), 1);

 	ASSERT_STRING_EQ(k2->cert->signature_type, "ssh-rsa");

 	sshkey_free(k2);

-	TEST_DONE();

+	TEST_DONE(); */

 	TEST_START("load RSA cert with SHA512 signature");

 	ASSERT_INT_EQ(sshkey_load_cert(test_data_file("rsa_1_sha512"), &k2), 0);

diff -up openssh-8.7p1/regress/unittests/sshkey/test_fuzz.c.sshrsacheck openssh-8.7p1/regress/unittests/sshkey/test_fuzz.c

--- openssh-8.7p1/regress/unittests/sshkey/test_fuzz.c.sshrsacheck	2023-01-26 12:10:37.533168013 +0100

+++ openssh-8.7p1/regress/unittests/sshkey/test_fuzz.c	2023-01-26 12:15:35.637631860 +0100

@@ -333,13 +333,14 @@ sshkey_fuzz_tests(void)

 	TEST_DONE();

 #ifdef WITH_OPENSSL

+	/* Skip this test, SHA1 signatures are not supported

 	TEST_START("fuzz RSA sig");

 	buf = load_file("rsa_1");

 	ASSERT_INT_EQ(sshkey_parse_private_fileblob(buf, "", &k1, NULL), 0);

 	sshbuf_free(buf);

 	sig_fuzz(k1, "ssh-rsa");

 	sshkey_free(k1);

-	TEST_DONE();

+	TEST_DONE();*/

 	TEST_START("fuzz RSA SHA256 sig");

 	buf = load_file("rsa_1");

diff -up openssh-8.7p1/regress/unittests/sshkey/test_sshkey.c.sshrsacheck openssh-8.7p1/regress/unittests/sshkey/test_sshkey.c

--- openssh-8.7p1/regress/unittests/sshkey/test_sshkey.c.sshrsacheck	2023-01-26 11:02:52.339413463 +0100

+++ openssh-8.7p1/regress/unittests/sshkey/test_sshkey.c	2023-01-26 11:58:42.324253896 +0100

@@ -60,6 +60,9 @@ build_cert(struct sshbuf *b, struct sshk

 	u_char *sigblob;

 	size_t siglen;

+	/* ssh-rsa implies SHA1, forbidden in DEFAULT cp */

+	int expected = (sig_alg == NULL || strcmp(sig_alg, "ssh-rsa") == 0) ? SSH_ERR_LIBCRYPTO_ERROR : 0;

+

 	ca_buf = sshbuf_new();

 	ASSERT_PTR_NE(ca_buf, NULL);

 	ASSERT_INT_EQ(sshkey_putb(ca_key, ca_buf), 0);

@@ -101,8 +104,9 @@ build_cert(struct sshbuf *b, struct sshk

 	ASSERT_INT_EQ(sshbuf_put_string(b, NULL, 0), 0); /* reserved */

 	ASSERT_INT_EQ(sshbuf_put_stringb(b, ca_buf), 0); /* signature key */

 	ASSERT_INT_EQ(sshkey_sign(sign_key, &sigblob, &siglen,

-	    sshbuf_ptr(b), sshbuf_len(b), sig_alg, NULL, NULL, 0), 0);

-	ASSERT_INT_EQ(sshbuf_put_string(b, sigblob, siglen), 0); /* signature */

+	    sshbuf_ptr(b), sshbuf_len(b), sig_alg, NULL, NULL, 0), expected);

+	if (expected == 0)

+		ASSERT_INT_EQ(sshbuf_put_string(b, sigblob, siglen), 0); /* signature */

 	free(sigblob);

 	sshbuf_free(ca_buf);

@@ -119,16 +123,22 @@ signature_test(struct sshkey *k, struct

 {

 	size_t len;

 	u_char *sig;

+	/* ssh-rsa implies SHA1, forbidden in DEFAULT cp in RHEL, permitted in Fedora */

+	int expected = (sig_alg && strcmp(sig_alg, "ssh-rsa") == 0) ? sshkey_sign(k, &sig, &len, d, l, sig_alg, NULL, NULL, 0) : 0;

+	if (k && (sshkey_type_plain(k->type) == KEY_DSA || sshkey_type_plain(k->type) == KEY_DSA_CERT))

+		expected = sshkey_sign(k, &sig, &len, d, l, sig_alg, NULL, NULL, 0);

 	ASSERT_INT_EQ(sshkey_sign(k, &sig, &len, d, l, sig_alg,

-	    NULL, NULL, 0), 0);

-	ASSERT_SIZE_T_GT(len, 8);

-	ASSERT_PTR_NE(sig, NULL);

-	ASSERT_INT_EQ(sshkey_verify(k, sig, len, d, l, NULL, 0, NULL), 0);

-	ASSERT_INT_NE(sshkey_verify(bad, sig, len, d, l, NULL, 0, NULL), 0);

-	/* Fuzz test is more comprehensive, this is just a smoke test */

-	sig[len - 5] ^= 0x10;

-	ASSERT_INT_NE(sshkey_verify(k, sig, len, d, l, NULL, 0, NULL), 0);

+	    NULL, NULL, 0), expected);

+	if (expected == 0) {

+		ASSERT_SIZE_T_GT(len, 8);

+		ASSERT_PTR_NE(sig, NULL);

+		ASSERT_INT_EQ(sshkey_verify(k, sig, len, d, l, NULL, 0, NULL), 0);

+		ASSERT_INT_NE(sshkey_verify(bad, sig, len, d, l, NULL, 0, NULL), 0);

+		/* Fuzz test is more comprehensive, this is just a smoke test */

+		sig[len - 5] ^= 0x10;

+		ASSERT_INT_NE(sshkey_verify(k, sig, len, d, l, NULL, 0, NULL), 0);

+	}

 	free(sig);

 }

@@ -514,7 +524,7 @@ sshkey_tests(void)

 	ASSERT_INT_EQ(sshkey_load_public(test_data_file("rsa_1.pub"), &k2,

 	    NULL), 0);

 	k3 = get_private("rsa_1");

-	build_cert(b, k2, "ssh-rsa-cert-v01@openssh.com", k3, k1, NULL);

+	build_cert(b, k2, "ssh-rsa-cert-v01@openssh.com", k3, k1, "rsa-sha2-256");

 	ASSERT_INT_EQ(sshkey_from_blob(sshbuf_ptr(b), sshbuf_len(b), &k4),

 	    SSH_ERR_KEY_CERT_INVALID_SIGN_KEY);

 	ASSERT_PTR_EQ(k4, NULL);

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

--- openssh-8.7p1/serverloop.c.sshrsacheck	2023-01-12 14:57:08.118400073 +0100

+++ openssh-8.7p1/serverloop.c	2023-01-12 14:59:17.330470518 +0100

@@ -80,6 +80,7 @@

 #include "auth-options.h"

 #include "serverloop.h"

 #include "ssherr.h"

+#include "compat.h"

 extern ServerOptions options;

@@ -737,6 +737,10 @@ server_input_hostkeys_prove(struct ssh *

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

 				sigalg = "rsa-sha2-256";

 		}

+		if (ssh->compat & SSH_RH_RSASIGSHA && sigalg == NULL) {

+			sigalg = "rsa-sha2-512";

+			debug3_f("SHA1 signature is not supported, falling back to %s", sigalg);

+		}

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

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

 		if ((r = sshbuf_put_cstring(sigbuf,

diff -up openssh-8.7p1/sshconnect2.c.sshrsacheck openssh-8.7p1/sshconnect2.c

--- openssh-8.7p1/sshconnect2.c.sshrsacheck	2023-01-25 15:33:29.140353651 +0100

+++ openssh-8.7p1/sshconnect2.c	2023-01-25 15:59:34.225364883 +0100

@@ -1461,6 +1464,14 @@ identity_sign(struct identity *id, u_cha

 			retried = 1;

 			goto retry_pin;

 		}

+		if ((r == SSH_ERR_LIBCRYPTO_ERROR) && strcmp("ssh-rsa", alg)) {

+			char rsa_safe_alg[] = "rsa-sha2-512";

+			debug3_f("trying to fallback to algorithm %s", rsa_safe_alg);

+

+			if ((r = sshkey_sign(sign_key, sigp, lenp, data, datalen,

+			rsa_safe_alg, options.sk_provider, pin, compat)) != 0)

+				debug_fr(r, "sshkey_sign - RSA fallback");

+		}

 		goto out;

 	}

diff -up openssh-8.7p1/ssh-rsa.c.sshrsacheck openssh-8.7p1/ssh-rsa.c

--- openssh-8.7p1/ssh-rsa.c.sshrsacheck	2023-01-20 13:07:54.180676144 +0100

+++ openssh-8.7p1/ssh-rsa.c	2023-01-20 13:07:54.290677074 +0100

@@ -254,7 +254,8 @@ ssh_rsa_verify(const struct sshkey *key,

 			ret = SSH_ERR_INVALID_ARGUMENT;

 			goto out;

 		}

-		if (hash_alg != want_alg) {

+		if (hash_alg != want_alg && want_alg != SSH_DIGEST_SHA1) {

+			debug_f("Unexpected digest algorithm: got %d, wanted %d", hash_alg, want_alg);

 			ret = SSH_ERR_SIGNATURE_INVALID;

 			goto out;

 		}

diff -up openssh-9.8p1/sshd-session.c.xxx openssh-9.8p1/sshd-session.c

--- openssh-9.8p1/sshd-session.c.xxx	2024-07-23 15:08:14.794350818 +0200

+++ openssh-9.8p1/sshd-session.c	2024-07-23 15:40:21.658456636 +0200

@@ -1305,6 +1305,27 @@ main(int ac, char **av)

 	check_ip_options(ssh);

+	{

+		struct sshkey *rsakey = NULL;

+		rsakey = get_hostkey_private_by_type(KEY_RSA, 0, ssh);

+		if (rsakey == NULL)

+			rsakey = get_hostkey_private_by_type(KEY_RSA_CERT, 0, ssh);

+

+		if (rsakey != NULL) {

+		    size_t sign_size = 0;

+		    u_char *tmp = NULL;

+		    u_char data[] = "Test SHA1 vector";

+		    int res;

+

+		    res = sshkey_sign(rsakey, &tmp, &sign_size, data, sizeof(data), NULL, NULL, NULL, 0);

+		    free(tmp);

+		    if (res == SSH_ERR_LIBCRYPTO_ERROR) {

+			verbose_f("SHA1 in signatures is disabled for RSA keys");

+		    	ssh->compat |= SSH_RH_RSASIGSHA;

+		    }

+		}

+	}

+

 	/* Prepare the channels layer */

 	channel_init_channels(ssh);

 	channel_set_af(ssh, options.address_family);