1 files changed, 22 insertions, 0 deletions
diff --git a/src/support/string.cpp b/src/support/string.cpp
index 31d0e9170..01fe4e522 100644
--- a/src/support/string.cpp
+++ b/src/support/string.cpp
@@ -195,9 +195,21 @@ std::optional<uint32_t> takeWTF8CodePoint(std::string_view& str) {
   }
 
   str = str.substr(1 + trailingBytes);
+
   if (!valid) {
     return std::nullopt;
   }
+
+  size_t expectedTrailing = u < 0x80       ? 0
+                            : u < 0x800    ? 1
+                            : u < 0x10000  ? 2
+                            : u < 0x110000 ? 3
+                                           : -1;
+  if (trailingBytes != expectedTrailing) {
+    // Overlong encoding or overlarge code point.
+    return std::nullopt;
+  }
+
   return u;
 }
 
@@ -404,4 +416,14 @@ std::ostream& printEscapedJSON(std::ostream& os, std::string_view str) {
   return os << '"';
 }
 
+bool isUTF8(std::string_view str) {
+  while (str.size()) {
+    auto u = takeWTF8CodePoint(str);
+    if (!u || (0xD800 <= *u && *u < 0xE000)) {
+      return false;
+    }
+  }
+  return true;
+}
+
 } // namespace wasm::String