parser: Reject invalid surogate pairs more consistently.

CHANGES.md

@@ -16,6 +16,7 @@
 * Fix `JSON.generate` `strict: true` mode to also restrict hash keys.
 * Fix `JSON::Coder` to also invoke block for hash keys that aren't strings nor symbols.
 * Fix `JSON.unsafe_load` usage with proc
+* Fix the parser to more consistently reject invalid UTF-16 surogate pairs. 
 
 ### 2025-07-28 (2.13.2)
 

ext/json/ext/parser/parser.c

@@ -713,11 +713,16 @@ static VALUE json_string_unescape(JSON_ParserState *state, const char *string, c
                         }
                         if (pe[0] == '\\' && pe[1] == 'u') {
                             uint32_t sur = unescape_unicode(state, (unsigned char *) pe + 2);
+
+                            if ((sur & 0xFC00) != 0xDC00) {
+                                raise_parse_error_at("invalid surrogate pair at %s", state, p);
+                            }
+
                             ch = (((ch & 0x3F) << 10) | ((((ch >> 6) & 0xF) + 1) << 16)
                                     | (sur & 0x3FF));
                             pe += 5;
                         } else {
-                            unescape = (char *) "?";
+                            raise_parse_error_at("incomplete surrogate pair at %s", state, p);
                             break;
                         }
                     }

test/json/json_parser_test.rb

@@ -319,6 +319,12 @@ def test_invalid_unicode_escape
     assert_raise(JSON::ParserError) { parse('"\u111___"') }
   end
 
+  def test_invalid_surogates
+    assert_raise(JSON::ParserError) { parse('"\\uD800"') }
+    assert_raise(JSON::ParserError) { parse('"\\uD800_________________"') }
+    assert_raise(JSON::ParserError) { parse('"\\uD800\\u0041"') }
+  end
+
   def test_parse_big_integers
     json1 = JSON(orig = (1 << 31) - 1)
     assert_equal orig, parse(json1)