Backport of:

From fd1c7e131f331942d20f42f31e79912d570081fa Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Thu, 2 Apr 2026 07:48:13 +0000
Subject: [PATCH] upstream: correctly match ECDSA signature algorithms against

algorithm allowlists: HostKeyAlgorithms, PubkeyAcceptedAlgorithms and
HostbasedAcceptedAlgorithms.

Previously, if any ECDSA type (say "ecdsa-sha2-nistp521") was
present in one of these lists, then all ECDSA algorithms would
be permitted.

Reported by Christos Papakonstantinou of Cantina and Spearbit.

OpenBSD-Commit-ID: c790e2687c35989ae34a00e709be935c55b16a86
---
 auth2-hostbased.c  |  9 +++++----
 auth2-pubkey.c     |  9 +++++----
 auth2-pubkeyfile.c | 26 +++++++++++++++-----------
 sshconnect2.c      | 28 ++++++++++++++++++----------
 4 files changed, 43 insertions(+), 29 deletions(-)

--- a/auth2-hostbased.c
+++ b/auth2-hostbased.c
@@ -96,9 +96,10 @@ userauth_hostbased(struct ssh *ssh, cons
 		error_f("cannot decode key: %s", pkalg);
 		goto done;
 	}
-	if (key->type != pktype) {
-		error_f("type mismatch for decoded key "
-		    "(received %d, expected %d)", key->type, pktype);
+	if (key->type != pktype || (sshkey_type_plain(pktype) == KEY_ECDSA &&
+	    sshkey_ecdsa_nid_from_name(pkalg) != key->ecdsa_nid)) {
+		error_f("key type mismatch for decoded key "
+		    "(received %s, expected %s)", sshkey_ssh_name(key), pkalg);
 		goto done;
 	}
 	if (sshkey_type_plain(key->type) == KEY_RSA &&
--- a/auth2-pubkey.c
+++ b/auth2-pubkey.c
@@ -67,6 +67,7 @@
 #include "authfile.h"
 #include "match.h"
 #include "ssherr.h"
+#include "xmalloc.h"
 #include "kex.h"
 #include "channels.h" /* XXX for session.h */
 #include "session.h" /* XXX for child_set_env(); refactor? */
@@ -150,9 +151,10 @@ userauth_pubkey(struct ssh *ssh, const c
 		error_f("cannot decode key: %s", pkalg);
 		goto done;
 	}
-	if (key->type != pktype) {
-		error_f("type mismatch for decoded key "
-		    "(received %d, expected %d)", key->type, pktype);
+	if (key->type != pktype || (sshkey_type_plain(pktype) == KEY_ECDSA &&
+	    sshkey_ecdsa_nid_from_name(pkalg) != key->ecdsa_nid)) {
+		error_f("key type mismatch for decoded key "
+		    "(received %s, expected %s)", sshkey_ssh_name(key), pkalg);
 		goto done;
 	}
 	if (sshkey_type_plain(key->type) == KEY_RSA &&
@@ -319,20 +321,23 @@ done:
 static int
 match_principals_option(const char *principal_list, struct sshkey_cert *cert)
 {
-	char *result;
+	char *list, *olist, *entry;
 	u_int i;
 
-	/* XXX percent_expand() sequences for authorized_principals? */
-
-	for (i = 0; i < cert->nprincipals; i++) {
-		if ((result = match_list(cert->principals[i],
-		    principal_list, NULL)) != NULL) {
-			debug3("matched principal from key options \"%.100s\"",
-			    result);
-			free(result);
-			return 1;
+	olist = list = xstrdup(principal_list);
+	for (;;) {
+		if ((entry = strsep(&list, ",")) == NULL || *entry == '\0')
+			break;
+		for (i = 0; i < cert->nprincipals; i++) {
+			if (strcmp(entry, cert->principals[i]) == 0) {
+				debug3("matched principal from key i"
+				    "options \"%.100s\"", entry);
+				free(olist);
+				return 1;
+			}
 		}
 	}
+	free(olist);
 	return 0;
 }
 
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -90,10 +90,15 @@ extern Options options;
 static char *xxx_host;
 static struct sockaddr *xxx_hostaddr;
 static const struct ssh_conn_info *xxx_conn_info;
+static int key_type_allowed(struct sshkey *, const char *);
 
 static int
 verify_host_key_callback(struct sshkey *hostkey, struct ssh *ssh)
 {
+	if (!key_type_allowed(hostkey, options.hostkeyalgorithms)) {
+		fatal("Server host key %s not in HostKeyAlgorithms",
+		    sshkey_ssh_name(hostkey));
+	}
 	if (verify_host_key(xxx_host, xxx_hostaddr, hostkey,
 	    xxx_conn_info) != 0)
 		fatal("Host key verification failed.");
@@ -1763,34 +1768,37 @@ load_identity_file(Identity *id)
 }
 
 static int
-key_type_allowed_by_config(struct sshkey *key)
+key_type_allowed(struct sshkey *key, const char *allowlist)
 {
-	if (match_pattern_list(sshkey_ssh_name(key),
-	    options.pubkey_accepted_algos, 0) == 1)
+	if (match_pattern_list(sshkey_ssh_name(key), allowlist, 0) == 1)
 		return 1;
 
 	/* RSA keys/certs might be allowed by alternate signature types */
 	switch (key->type) {
 	case KEY_RSA:
-		if (match_pattern_list("rsa-sha2-512",
-		    options.pubkey_accepted_algos, 0) == 1)
+		if (match_pattern_list("rsa-sha2-512", allowlist, 0) == 1)
 			return 1;
-		if (match_pattern_list("rsa-sha2-256",
-		    options.pubkey_accepted_algos, 0) == 1)
+		if (match_pattern_list("rsa-sha2-256", allowlist, 0) == 1)
 			return 1;
 		break;
 	case KEY_RSA_CERT:
 		if (match_pattern_list("rsa-sha2-512-cert-v01@openssh.com",
-		    options.pubkey_accepted_algos, 0) == 1)
+		    allowlist, 0) == 1)
 			return 1;
 		if (match_pattern_list("rsa-sha2-256-cert-v01@openssh.com",
-		    options.pubkey_accepted_algos, 0) == 1)
+		    allowlist, 0) == 1)
 			return 1;
 		break;
 	}
 	return 0;
 }
 
+static int
+key_type_allowed_by_config(struct sshkey *key)
+{
+	return key_type_allowed(key, options.pubkey_accepted_algos);
+}
+
 /* obtain a list of keys from the agent */
 static int
 get_agent_identities(struct ssh *ssh, int *agent_fdp,
