diff --color -rup a/compat.c b/compat.c

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

+++ b/compat.c	2022-07-11 10:00:56.661195753 +0200

@@ -157,11 +157,12 @@ compat_banner(struct ssh *ssh, const cha

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

 }

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

 char *

 compat_cipher_proposal(struct ssh *ssh, char *cipher_prop)

 {

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

-		return cipher_prop;

+		return xstrdup(cipher_prop);

 	debug2_f("original cipher proposal: %s", cipher_prop);

 	if ((cipher_prop = match_filter_denylist(cipher_prop, "aes*")) == NULL)

 		fatal("match_filter_denylist failed");

@@ -171,11 +172,12 @@ compat_cipher_proposal(struct ssh *ssh,

 	return cipher_prop;

 }

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

 char *

 compat_pkalg_proposal(struct ssh *ssh, char *pkalg_prop)

 {

 	if (!(ssh->compat & SSH_BUG_RSASIGMD5))

-		return pkalg_prop;

+		return xstrdup(pkalg_prop);

 	debug2_f("original public key proposal: %s", pkalg_prop);

 	if ((pkalg_prop = match_filter_denylist(pkalg_prop, "ssh-rsa")) == NULL)

 		fatal("match_filter_denylist failed");

@@ -185,21 +187,26 @@ compat_pkalg_proposal(struct ssh *ssh, c

 	return pkalg_prop;

 }

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

 char *

 compat_kex_proposal(struct ssh *ssh, char *p)

 {

+	char *cp = NULL;

+

 	if ((ssh->compat & (SSH_BUG_CURVE25519PAD|SSH_OLD_DHGEX)) == 0)

-		return p;

+		return xstrdup(p);

 	debug2_f("original KEX proposal: %s", p);

 	if ((ssh->compat & SSH_BUG_CURVE25519PAD) != 0)

 		if ((p = match_filter_denylist(p,

 		    "curve25519-sha256@libssh.org")) == NULL)

 			fatal("match_filter_denylist failed");

 	if ((ssh->compat & SSH_OLD_DHGEX) != 0) {

+		cp = p;

 		if ((p = match_filter_denylist(p,

 		    "diffie-hellman-group-exchange-sha256,"

 		    "diffie-hellman-group-exchange-sha1")) == NULL)

 			fatal("match_filter_denylist failed");

+		free(cp);

 	}

 	debug2_f("compat KEX proposal: %s", p);

 	if (*p == '\0')

diff --color -rup a/sshconnect2.c b/sshconnect2.c

--- a/sshconnect2.c	2022-07-11 09:54:22.300523575 +0200

+++ b/sshconnect2.c	2022-07-11 10:13:20.516655403 +0200

@@ -218,6 +218,7 @@ ssh_kex2(struct ssh *ssh, char *host, st

 {

 	char *myproposal[PROPOSAL_MAX] = { KEX_CLIENT };

 	char *s, *all_key;

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

 	int r, use_known_hosts_order = 0;

 #if defined(GSSAPI) && defined(WITH_OPENSSL)

@@ -248,10 +249,9 @@ ssh_kex2(struct ssh *ssh, char *host, st

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

 		fatal_f("kex_names_cat");

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

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

 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =

-	    compat_cipher_proposal(ssh, options.ciphers);

-	myproposal[PROPOSAL_ENC_ALGS_STOC] =

+	    myproposal[PROPOSAL_ENC_ALGS_STOC] = prop_enc =

 	    compat_cipher_proposal(ssh, options.ciphers);

 	myproposal[PROPOSAL_COMP_ALGS_CTOS] =

 	    myproposal[PROPOSAL_COMP_ALGS_STOC] =

@@ -260,12 +260,12 @@ ssh_kex2(struct ssh *ssh, char *host, st

 	    myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;

 	if (use_known_hosts_order) {

 		/* Query known_hosts and prefer algorithms that appear there */

-		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =

+		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = prop_hostkey =

 		    compat_pkalg_proposal(ssh,

 		    order_hostkeyalgs(host, hostaddr, port, cinfo));

 	} else {

 		/* Use specified HostkeyAlgorithms exactly */

-		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =

+		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = prop_hostkey =

 		    compat_pkalg_proposal(ssh, options.hostkeyalgorithms);

 	}

@@ -379,6 +379,10 @@ ssh_kex2(struct ssh *ssh, char *host, st

 	    (r = ssh_packet_write_wait(ssh)) != 0)

 		fatal_fr(r, "send packet");

 #endif

+	/* Free only parts of proposal that were dynamically allocated here. */

+	free(prop_kex);

+	free(prop_enc);

+	free(prop_hostkey);

 }

 /*

diff --color -rup a/sshd.c b/sshd.c

--- a/sshd.c	2022-07-11 09:54:22.301523594 +0200

+++ b/sshd.c	2022-07-11 10:17:15.692209542 +0200

@@ -2479,14 +2479,14 @@ do_ssh2_kex(struct ssh *ssh)

 {

 	char *myproposal[PROPOSAL_MAX] = { KEX_SERVER };

 	struct kex *kex;

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

 	int r;

-	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(ssh,

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

 	    options.kex_algorithms);

-	myproposal[PROPOSAL_ENC_ALGS_CTOS] = compat_cipher_proposal(ssh,

-	    options.ciphers);

-	myproposal[PROPOSAL_ENC_ALGS_STOC] = compat_cipher_proposal(ssh,

-	    options.ciphers);

+	myproposal[PROPOSAL_ENC_ALGS_CTOS] =

+	    myproposal[PROPOSAL_ENC_ALGS_STOC] = prop_enc =

+	    compat_cipher_proposal(ssh, options.ciphers);

 	myproposal[PROPOSAL_MAC_ALGS_CTOS] =

 	    myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;

@@ -2499,8 +2499,8 @@ do_ssh2_kex(struct ssh *ssh)

 		ssh_packet_set_rekey_limits(ssh, options.rekey_limit,

 		    options.rekey_interval);

-	myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = compat_pkalg_proposal(

-	    ssh, list_hostkey_types());

+	myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = prop_hostkey =

+	    compat_pkalg_proposal(ssh, list_hostkey_types());

 #if defined(GSSAPI) && defined(WITH_OPENSSL)

 	{

@@ -2592,6 +2592,9 @@ do_ssh2_kex(struct ssh *ssh)

 	    (r = ssh_packet_write_wait(ssh)) != 0)

 		fatal_fr(r, "send test");

 #endif

+	free(prop_kex);

+	free(prop_enc);

+	free(prop_hostkey);

 	debug("KEX done");

 }