From 8744d058ea338bf8a0dfd8f2d21e6120ec386f81 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Sat, 27 Jun 2026 11:41:23 +0200
Subject: [PATCH] [3.13] gh-152212: Reject a POSIX TZ footer with a missing std
 offset in pure-Python `zoneinfo` (GH-152213) (#152377)
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(cherry picked from commit 93454fe0daa2b29e8fefbe76aed7af4fa1eb8623)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
---
 Lib/test/test_zoneinfo/test_zoneinfo.py                      | 5 +++++
 Lib/zoneinfo/_zoneinfo.py                                    | 3 ++-
 .../Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst   | 3 +++
 3 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst

diff --git a/Lib/test/test_zoneinfo/test_zoneinfo.py b/Lib/test/test_zoneinfo/test_zoneinfo.py
index 21033882e14..323fc845aa4 100644
--- a/Lib/test/test_zoneinfo/test_zoneinfo.py
+++ b/Lib/test/test_zoneinfo/test_zoneinfo.py
@@ -1142,6 +1142,11 @@ def test_extreme_tzstr(self):
     def test_invalid_tzstr(self):
         invalid_tzstrs = [
             "PST8PDT",  # DST but no transition specified
+            # gh-152212: the std offset is required (POSIX TZ grammar)
+            "AAA",
+            "A",
+            "AA",
+            "B",
             "+11",  # Unquoted alphanumeric
             "GMT,M3.2.0/2,M11.1.0/3",  # Transition rule but no DST
             "GMT0+11,M3.2.0/2,M11.1.0/3",  # Unquoted alphanumeric in DST
diff --git a/Lib/zoneinfo/_zoneinfo.py b/Lib/zoneinfo/_zoneinfo.py
index 3903d57d554..52832f600c3 100644
--- a/Lib/zoneinfo/_zoneinfo.py
+++ b/Lib/zoneinfo/_zoneinfo.py
@@ -672,7 +672,8 @@ def _parse_tz_str(tz_str):
         except ValueError as e:
             raise ValueError(f"Invalid STD offset in {tz_str}") from e
     else:
-        std_offset = 0
+        # The STD offset is required
+        raise ValueError(f"Invalid STD offset in {tz_str}")
 
     if dst_abbr is not None:
         if dst_offset := m.group("dstoff"):
diff --git a/Misc/NEWS.d/next/Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst b/Misc/NEWS.d/next/Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst
new file mode 100644
index 00000000000..2b1d272638b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst
@@ -0,0 +1,3 @@
+Fix the pure-Python :mod:`zoneinfo` parser accepting a POSIX TZ string with a
+``std`` abbreviation but no offset. This is invalid per POSIX and now
+raises :exc:`ValueError`, matching the C accelerator. Patch by tonghuaroot.