diff --git a/misc.c b/misc.c
index e77b869be..253ecd5bd 100644
--- a/misc.c
+++ b/misc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: misc.c,v 1.183 2023/07/14 07:44:21 dtucker Exp $ */
+/* $OpenBSD: misc.c,v 1.184 2023/07/19 14:02:27 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2005-2020 Damien Miller.  All rights reserved.
@@ -22,6 +22,7 @@
 
 #include <sys/types.h>
 #include <sys/ioctl.h>
+#include <sys/mman.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/time.h>
@@ -35,6 +36,9 @@
 #ifdef HAVE_POLL_H
 #include <poll.h>
 #endif
+#ifdef HAVE_NLIST_H
+#include <nlist.h>
+#endif
 #include <signal.h>
 #include <stdarg.h>
 #include <stdio.h>
@@ -2920,3 +2924,75 @@ ptimeout_isset(struct timespec *pt)
 {
 	return pt->tv_sec != -1;
 }
+
+/*
+ * Returns zero if the library at 'path' contains symbol 's', nonzero
+ * otherwise.
+ */
+int
+lib_contains_symbol(const char *path, const char *s)
+{
+#ifdef HAVE_NLIST_H
+	struct nlist nl[2];
+	int ret = -1, r;
+
+	memset(nl, 0, sizeof(nl));
+	nl[0].n_name = xstrdup(s);
+	nl[1].n_name = NULL;
+	if ((r = nlist(path, nl)) == -1) {
+		error_f("nlist failed for %s", path);
+		goto out;
+	}
+	if (r != 0 || nl[0].n_value == 0 || nl[0].n_type == 0) {
+		error_f("library %s does not contain symbol %s", path, s);
+		goto out;
+	}
+	/* success */
+	ret = 0;
+ out:
+	free(nl[0].n_name);
+	return ret;
+#else /* HAVE_NLIST_H */
+	int fd, ret = -1;
+	struct stat st;
+	void *m = NULL;
+	size_t sz = 0;
+
+	memset(&st, 0, sizeof(st));
+	if ((fd = open(path, O_RDONLY)) < 0) {
+		error_f("open %s: %s", path, strerror(errno));
+		return -1;
+	}
+	if (fstat(fd, &st) != 0) {
+		error_f("fstat %s: %s", path, strerror(errno));
+		goto out;
+	}
+	if (!S_ISREG(st.st_mode)) {
+		error_f("%s is not a regular file", path);
+		goto out;
+	}
+	if (st.st_size < 0 ||
+	    (size_t)st.st_size < strlen(s) ||
+	    st.st_size >= INT_MAX/2) {
+		error_f("%s bad size %lld", path, (long long)st.st_size);
+		goto out;
+	}
+	sz = (size_t)st.st_size;
+	if ((m = mmap(NULL, sz, PROT_READ, MAP_PRIVATE, fd, 0)) == MAP_FAILED ||
+	    m == NULL) {
+		error_f("mmap %s: %s", path, strerror(errno));
+		goto out;
+	}
+	if (memmem(m, sz, s, strlen(s)) == NULL) {
+		error_f("%s does not contain expected string %s", path, s);
+		goto out;
+	}
+	/* success */
+	ret = 0;
+ out:
+	if (m != NULL && m != MAP_FAILED)
+		munmap(m, sz);
+	close(fd);
+	return ret;
+#endif /* HAVE_NLIST_H */
+}
diff --git a/misc.h b/misc.h
index 07408ca13..fd77a7fd7 100644
--- a/misc.h
+++ b/misc.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: misc.h,v 1.102 2023/03/03 02:37:58 dtucker Exp $ */
+/* $OpenBSD: misc.h,v 1.103 2023/07/19 14:02:27 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -96,6 +96,7 @@ int	 parse_absolute_time(const char *, uint64_t *);
 void	 format_absolute_time(uint64_t, char *, size_t);
 int	 path_absolute(const char *);
 int	 stdfd_devnull(int, int, int);
+int	 lib_contains_symbol(const char *, const char *);
 
 void	 sock_set_v6only(int);
 
diff --git a/ssh-pkcs11.c b/ssh-pkcs11.c
index 0b51e7753..8e2b9cb97 100644
--- a/ssh-pkcs11.c
+++ b/ssh-pkcs11.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-pkcs11.c,v 1.57 2023/07/19 13:55:53 djm Exp $ */
+/* $OpenBSD: ssh-pkcs11.c,v 1.58 2023/07/19 14:02:27 djm Exp $ */
 /*
  * Copyright (c) 2010 Markus Friedl.  All rights reserved.
  * Copyright (c) 2014 Pedro Martelletto. All rights reserved.
@@ -1532,6 +1532,10 @@ pkcs11_register_provider(char *provider_id, char *pin,
 		debug_f("provider already registered: %s", provider_id);
 		goto fail;
 	}
+	if (lib_contains_symbol(provider_id, "C_GetFunctionList") != 0) {
+		error("provider %s is not a PKCS11 library", provider_id);
+		goto fail;
+	}
 	/* open shared pkcs11-library */
 	if ((handle = dlopen(provider_id, RTLD_NOW)) == NULL) {
 		error("dlopen %s failed: %s", provider_id, dlerror());
diff --git a/ssh-sk.c b/ssh-sk.c
index fbeb39320..d1c18803f 100644
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.c,v 1.39 2022/07/20 03:29:14 djm Exp $ */
+/* $OpenBSD: ssh-sk.c,v 1.40 2023/07/19 14:02:27 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -133,10 +133,12 @@ sshsk_open(const char *path)
 		goto fail;
 #endif
 	}
-	if ((ret->dlhandle = dlopen(path, RTLD_NOW)) == NULL) {
-		error("Provider \"%s\" dlopen failed: %s", path, dlerror());
+	if (lib_contains_symbol(path, "sk_api_version") != 0) {
+		error("provider %s is not an OpenSSH FIDO library", path);
 		goto fail;
 	}
+	if ((ret->dlhandle = dlopen(path, RTLD_NOW)) == NULL)
+		fatal("Provider \"%s\" dlopen failed: %s", path, dlerror());
 	if ((ret->sk_api_version = dlsym(ret->dlhandle,
 	    "sk_api_version")) == NULL) {
 		error("Provider \"%s\" dlsym(sk_api_version) failed: %s",