diff --git a/src/tls/tlsv1_client_read.c b/src/tls/tlsv1_client_read.c
index 3825a73..9df56c2 100644
--- a/src/tls/tlsv1_client_read.c
+++ b/src/tls/tlsv1_client_read.c
@@ -771,7 +771,8 @@
 			hlen = tls_key_x_server_params_hash(
 				conn->rl.tls_version, conn->client_random,
 				conn->server_random, server_params,
-				server_params_end - server_params, hash);
+				server_params_end - server_params, hash,
+				sizeof(hash));
 		}
 
 		if (hlen < 0)
diff --git a/src/tls/tlsv1_common.c b/src/tls/tlsv1_common.c
index e178915..0dd8e27 100644
--- a/src/tls/tlsv1_common.c
+++ b/src/tls/tlsv1_common.c
@@ -378,7 +378,7 @@
 int tls_key_x_server_params_hash(u16 tls_version, const u8 *client_random,
 				 const u8 *server_random,
 				 const u8 *server_params,
-				 size_t server_params_len, u8 *hash)
+				 size_t server_params_len, u8 *hash, size_t hsz)
 {
 	u8 *hpos;
 	size_t hlen;
@@ -393,6 +393,8 @@
 	crypto_hash_update(ctx, server_random, TLS_RANDOM_LEN);
 	crypto_hash_update(ctx, server_params, server_params_len);
 	hlen = MD5_MAC_LEN;
+	if (hsz < hlen)
+		return -1;
 	if (crypto_hash_finish(ctx, hash, &hlen) < 0)
 		return -1;
 	hpos += hlen;
@@ -403,7 +405,7 @@
 	crypto_hash_update(ctx, client_random, TLS_RANDOM_LEN);
 	crypto_hash_update(ctx, server_random, TLS_RANDOM_LEN);
 	crypto_hash_update(ctx, server_params, server_params_len);
-	hlen = hash + sizeof(hash) - hpos;
+	hlen = hsz - hlen;
 	if (crypto_hash_finish(ctx, hpos, &hlen) < 0)
 		return -1;
 	hpos += hlen;
diff --git a/src/tls/tlsv1_common.h b/src/tls/tlsv1_common.h
index e30b15a..4cfdc2d 100644
--- a/src/tls/tlsv1_common.h
+++ b/src/tls/tlsv1_common.h
@@ -267,7 +267,8 @@
 int tls_key_x_server_params_hash(u16 tls_version, const u8 *client_random,
 				 const u8 *server_random,
 				 const u8 *server_params,
-				 size_t server_params_len, u8 *hash);
+				 size_t server_params_len,
+				 u8 *hash, size_t hsz);
 int tls_verify_signature(u16 tls_version, struct crypto_public_key *pk,
 			 const u8 *data, size_t data_len,
 			 const u8 *pos, size_t len, u8 *alert);
diff --git a/src/tls/tlsv1_server_write.c b/src/tls/tlsv1_server_write.c
index 8d36cf1..545abae 100644
--- a/src/tls/tlsv1_server_write.c
+++ b/src/tls/tlsv1_server_write.c
@@ -620,7 +620,7 @@
 			hlen = tls_key_x_server_params_hash(
 				conn->rl.tls_version, conn->client_random,
 				conn->server_random, server_params,
-				pos - server_params, hash);
+				pos - server_params, hash, sizeof(hash));
 		}
 
 		if (hlen < 0) {