From f3e6bdafd2804c1bf7d4cb31fed3f983fd5936ba Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Fri, 26 Jun 2026 16:15:21 +0200
Subject: [PATCH] [3.15] gh-152052: Fix misleading `json` error for `\uXXXX`
 escape at the end of input (GH-152053) (#152283)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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(cherry picked from commit 588be7af08c04d63c1a920539c64885fbc4c6d38)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>
---
 Lib/test/test_json/test_fail.py                            | 7 +++++++
 Lib/test/test_json/test_scanstring.py                      | 3 +++
 .../Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst | 2 ++
 Modules/_json.c                                            | 2 +-
 4 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst

diff --git a/Lib/test/test_json/test_fail.py b/Lib/test/test_json/test_fail.py
index 79c44af2fbf..7f99a6331d3 100644
--- a/Lib/test/test_json/test_fail.py
+++ b/Lib/test/test_json/test_fail.py
@@ -144,6 +144,13 @@ def test_truncated_input(self):
             ('"', 'Unterminated string starting at', 0),
             ('"spam', 'Unterminated string starting at', 0),
         ]
+        # A complete \uXXXX escape at end of input leaves it unterminated.
+        test_cases += [
+            (r'"\u0041', 'Unterminated string starting at', 0),
+            (r'"\ud834', 'Unterminated string starting at', 0),
+            (r'"\ud834\udd1e', 'Unterminated string starting at', 0),
+            (r'{"a": "\u0041', 'Unterminated string starting at', 6),
+        ]
         for data, msg, idx in test_cases:
             with self.assertRaises(self.JSONDecodeError) as cm:
                 self.loads(data)
diff --git a/Lib/test/test_json/test_scanstring.py b/Lib/test/test_json/test_scanstring.py
index 9a6cdfe12d2..a52b01c7113 100644
--- a/Lib/test/test_json/test_scanstring.py
+++ b/Lib/test/test_json/test_scanstring.py
@@ -137,6 +137,9 @@ def test_bad_escapes(self):
             '"\\ud834\\u-123"',
             '"\\ud834\\u+123"',
             '"\\ud834\\u1_23"',
+            # Truncated or non-hex \uXXXX escape at end of input.
+            '"\\u004',
+            '"\\uXYZW',
         ]
         for s in bad_escapes:
             with self.assertRaises(self.JSONDecodeError, msg=s):
diff --git a/Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst b/Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst
new file mode 100644
index 00000000000..f9ffbbd342f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst
@@ -0,0 +1,2 @@
+The :mod:`json` C accelerator now correctly reports an unterminated string for a
+``\uXXXX`` escape at the end of the input.
diff --git a/Modules/_json.c b/Modules/_json.c
index 6c4f3883463..858702e6f45 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -574,7 +574,7 @@ scanstring_unicode(PyObject *pystr, Py_ssize_t end, int strict, Py_ssize_t *next
             c = 0;
             next++;
             end = next + 4;
-            if (end >= len) {
+            if (end > len) {
                 raise_errmsg("Invalid \\uXXXX escape", pystr, next - 1);
                 goto bail;
             }