diff --git a/mongoose.c b/mongoose.c
index dd15b18e..79690134 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -433,9 +433,10 @@ int mg_http_get_var(const struct mg_str *buf, const char *name, char *dst,
 int mg_url_decode(const char *src, size_t src_len, char *dst, size_t dst_len,
                   int is_form_url_encoded) {
   size_t i, j;
-  for (i = j = 0; i < src_len && j < dst_len - 1; i++, j++) {
+  for (i = j = 0; i < src_len && j + 1 < dst_len; i++, j++) {
     if (src[i] == '%') {
-      if (i < src_len - 2 && isxdigit(*(const unsigned char *) (src + i + 1)) &&
+      // Use `i + 2 < src_len`, not `i < src_len - 2`, note small src_len
+      if (i + 2 < src_len && isxdigit(*(const unsigned char *) (src + i + 1)) &&
           isxdigit(*(const unsigned char *) (src + i + 2))) {
         mg_unhex(src + i + 1, 2, (uint8_t *) &dst[j]);
         i += 2;
@@ -448,7 +449,7 @@ int mg_url_decode(const char *src, size_t src_len, char *dst, size_t dst_len,
       dst[j] = src[i];
     }
   }
-  dst[j] = '\0';  // Null-terminate the destination
+  if (j < dst_len) dst[j] = '\0';  // Null-terminate the destination
   return i >= src_len ? (int) j : -1;
 }
 
@@ -1735,7 +1736,7 @@ static int parse(const uint8_t *in, size_t inlen, struct mqtt_message *m) {
     len += (lc & 0x7f) << 7 * len_len;
     len_len++;
     if (!(lc & 0x80)) break;
-    if (len_len > 4) return MQTT_MALFORMED;
+    if (len_len >= 4) return MQTT_MALFORMED;
   }
 
   end = p + len;
diff --git a/src/http.c b/src/http.c
index 95acaeb6..eb232e3e 100644
--- a/src/http.c
+++ b/src/http.c
@@ -73,9 +73,10 @@ int mg_http_get_var(const struct mg_str *buf, const char *name, char *dst,
 int mg_url_decode(const char *src, size_t src_len, char *dst, size_t dst_len,
                   int is_form_url_encoded) {
   size_t i, j;
-  for (i = j = 0; i < src_len && j < dst_len - 1; i++, j++) {
+  for (i = j = 0; i < src_len && j + 1 < dst_len; i++, j++) {
     if (src[i] == '%') {
-      if (i < src_len - 2 && isxdigit(*(const unsigned char *) (src + i + 1)) &&
+      // Use `i + 2 < src_len`, not `i < src_len - 2`, note small src_len
+      if (i + 2 < src_len && isxdigit(*(const unsigned char *) (src + i + 1)) &&
           isxdigit(*(const unsigned char *) (src + i + 2))) {
         mg_unhex(src + i + 1, 2, (uint8_t *) &dst[j]);
         i += 2;
@@ -88,7 +89,7 @@ int mg_url_decode(const char *src, size_t src_len, char *dst, size_t dst_len,
       dst[j] = src[i];
     }
   }
-  dst[j] = '\0';  // Null-terminate the destination
+  if (j < dst_len) dst[j] = '\0';  // Null-terminate the destination
   return i >= src_len ? (int) j : -1;
 }
 
diff --git a/src/mqtt.c b/src/mqtt.c
index 4167a97c..93abc77c 100644
--- a/src/mqtt.c
+++ b/src/mqtt.c
@@ -150,7 +150,7 @@ static int parse(const uint8_t *in, size_t inlen, struct mqtt_message *m) {
     len += (lc & 0x7f) << 7 * len_len;
     len_len++;
     if (!(lc & 0x80)) break;
-    if (len_len > 4) return MQTT_MALFORMED;
+    if (len_len >= 4) return MQTT_MALFORMED;
   }
 
   end = p + len;
diff --git a/test/fuzz.c b/test/fuzz.c
index bb6919e9..c8dd9316 100644
--- a/test/fuzz.c
+++ b/test/fuzz.c
@@ -11,6 +11,8 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   struct mg_str body = mg_str_n((const char *) data, size);
   char tmp[256];
   mg_http_get_var(&body, "key", tmp, sizeof(tmp));
+  mg_url_decode((char *) data, size, tmp, sizeof(tmp), 1);
+  mg_url_decode((char *) data, size, tmp, 1, 1);
 
   struct mg_mqtt_message mm;
   mg_mqtt_parse(data, size, &mm);
diff --git a/test/unit_test.c b/test/unit_test.c
index 1ba942d9..ad1a0b7f 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -92,6 +92,8 @@ static void test_http_get_var(void) {
   body = mg_str("key=broken%2x");
   ASSERT(mg_http_get_var(&body, "key", buf, sizeof(buf)) == -3);
   ASSERT(mg_http_get_var(&body, "inexistent", buf, sizeof(buf)) == -4);
+  body = mg_str("key=%");
+  ASSERT(mg_http_get_var(&body, "key", buf, sizeof(buf)) == -3);
 }
 
 static int vcmp(struct mg_str s1, const char *s2) {
@@ -270,6 +272,12 @@ static void test_mqtt(void) {
   int i;
   mg_mgr_init(&mgr);
 
+  {
+    uint8_t bad[] = " \xff\xff\xff\xff ";
+    struct mg_mqtt_message mm;
+    mg_mqtt_parse(bad, sizeof(bad), &mm);
+  }
+
   // Connect with empty client ID
   c = mg_mqtt_connect(&mgr, url, NULL, mqtt_cb, buf);
   for (i = 0; i < 100 && buf[0] == 0; i++) mg_mgr_poll(&mgr, 10);
@@ -859,12 +867,17 @@ static void test_util(void) {
   ASSERT(mg_file_write("data.txt", "%s", "hi") == 2);
   ASSERT(strcmp(mg_ntoa(0x100007f, buf, sizeof(buf)), "127.0.0.1") == 0);
   ASSERT(strcmp(mg_hex("abc", 3, buf), "616263") == 0);
+  {
+    char bad[] = {'a', '=', '%'};
+    ASSERT(mg_url_decode(bad, sizeof(bad), buf, sizeof(buf), 0) < 0);
+  }
 }
 
 int main(void) {
   mg_log_set("3");
-  test_str();
   test_util();
+  test_mqtt();
+  test_str();
   test_timer();
   test_http_range();
   test_url();
@@ -875,7 +888,6 @@ int main(void) {
   test_http_get_var();
   test_tls();
   test_ws();
-  test_mqtt();
   test_http_parse();
   test_http_server();
   test_http_client();