diff --git a/Lib/test/test_json/test_decode.py b/Lib/test/test_json/test_decode.py
index 35c02de88cc..591b2e253df 100644
--- a/Lib/test/test_json/test_decode.py
+++ b/Lib/test/test_json/test_decode.py
@@ -91,5 +91,9 @@ class TestDecode:
         self.assertEqual(self.loads(bom_in_str), '\ufeff')
         self.assertEqual(self.json.load(StringIO(bom_in_str)), '\ufeff')
 
+    def test_negative_index(self):
+        d = self.json.JSONDecoder()
+        self.assertRaises(ValueError, d.raw_decode, 'a'*42, -50000)
+
 class TestPyDecode(TestDecode, PyTest): pass
 class TestCDecode(TestDecode, CTest): pass
diff --git a/Misc/ACKS b/Misc/ACKS
index 437aa461dc3..463d752037a 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1373,6 +1373,7 @@ Johannes Vogel
 Alex Volkov
 Martijn Vries
 Sjoerd de Vries
+Guido Vranken
 Niki W. Waibel
 Wojtek Walczak
 Charles Waldman
diff --git a/Misc/NEWS b/Misc/NEWS
index a8da8ab2050..a363a14cc10 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -34,6 +34,9 @@ Core and Builtins
 Library
 -------
 
+- Fix arbitrary memory access in JSONDecoder.raw_decode with a negative second
+  parameter. Bug reported by Guido Vranken.
+
 - Issue #21169: getpass now handles non-ascii characters that the
   input stream encoding cannot encode by re-encoding using the
   replace error handler.
diff --git a/Modules/_json.c b/Modules/_json.c
index 125101fa7c3..7d627cb2c54 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -941,7 +941,10 @@ scan_once_unicode(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_
     kind = PyUnicode_KIND(pystr);
     length = PyUnicode_GET_LENGTH(pystr);
 
-    if (idx >= length) {
+    if (idx < 0)
+        /* Compatibility with Python version. */
+        idx += length;
+    if (idx < 0 || idx >= length) {
         raise_stop_iteration(idx);
         return NULL;
     }