gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` by StanFromIreland · Pull Request #157580 · python/cpython

StanFromIreland · 2026-09-15T18:12:03Z

Issue: Race condition in the cleanup of tempfile.TemporaryDirectory #157579

This reduces the attack surface -- readable files avoid the race condition.

Co-authored-by: Stan Ulbrych <stan@python.org>

read-the-docs-community · 2026-09-15T18:16:36Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #34695934 | 📁 Comparing 7568089 against main (82952e3)

🔍 Preview build

18 files changed · ± 18 modified

± Modified

StanFromIreland · 2026-09-15T19:33:57Z

I'm not sure why Emscripten failed in this run on test_cleanup_with_symlink_race:

 ======================================================================
FAIL: test_cleanup_with_symlink_race (test.test_tempfile.TestTemporaryDirectory.test_cleanup_with_symlink_race)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython/Lib/test/test_tempfile.py", line 1907, in test_cleanup_with_symlink_race
    self.assertTrue(os.path.exists(target_file))
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true

The test swaps a directory for a symlink to the outside directory mid-recovery, however the unlink followed the symlink and deleted it. This is odd because the unlink was passed a dir_fd for the directory that got swapped out. However, it doesn’t hit our skip (so the platform meets the requirements, which means os.unlink is indeed in os.supports_dir_fd). CC @hoodmane any idea what's going on here?

hoodmane · 2026-09-15T19:39:08Z

Will take a look.

encukou · 2026-09-16T07:48:02Z

Meanwhile let's start buildbots for all the other architectures/configurations

bedevere-bot · 2026-09-16T07:48:09Z

🤖 New build scheduled with the buildbot fleet by @encukou for commit a837624 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F157580%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

StanFromIreland · 2026-09-16T21:07:02Z

And it seems all the buildbots older with masOS <26 (and iOS) are failing on test_cleanup_with_symlink_race with:

======================================================================
ERROR: test_cleanup_with_symlink_race (test.test_tempfile.TestTemporaryDirectory.test_cleanup_with_symlink_race)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.nogil/build/Lib/test/test_tempfile.py", line 1904, in test_cleanup_with_symlink_race
    os.chmod(dir1, 0o700)
    ~~~~~~~~^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/test_python_lxxiashb/3w36qc28/dir1'

We don't know if the test passes or not, as we don't reach the final assertions. I'm also not quite sure why this happens, and I don't have access to a machine to debug. It's possible that the hook's rename of the 0o500 directory fails? Any ideas Petr?

encukou · 2026-09-17T16:29:30Z

Funny story

I can reproduce! .... on an ancient Intel MacBook that bought it in this state to take macOS screenshots for tutorials. It lasts ~10 min on battery and has a broken screen. It also has only has 1 USB port, so I can charge it, or connect a monitor/LAN, but not both at once. (This was never a problem for screenshots...)

So, work is slow. But, in the end, after several tries, I did manage to find a compatible dongle.

Turns out that on old macOS, you can't rename a directory if you don't have write permissions on that directory. (On Linux and -- apparently -- new macs, you need write on the containing directory; that has the inode you're manipulating.)

The rmtree will:

open d1 & start scandir
remove test0.txt from d1 via the monkeypatched hook; this succeeds
open dir1 & start scandir
remove file1 from dir1 via the monkeypatched hook
- remove file1 from dir1; this fails with PermissionError
- rename dir1 to dir1_moved; this also fails due to PermissionError (!)
call onexc with hook as func, file1 as path, dir1's fd as dir_fd
- call chflags on dir1
- call chmod on dir1's fd
- call chflags on file1
- call chmod on file1 via dir1's fd
- remove file1 (via the hook; this succeeds)
remove dir1 via d1's fd
rmdir d1 via path

Back in the test, in the finally:

islink is False as dir1 doesn't exist
chmod fails as dir1 doesn't exist

miss-islington-app · 2026-09-29T15:35:04Z

Sorry, @StanFromIreland and @encukou, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 5c20517a4fc56683efe63a7751020db9573f538d 3.11

miss-islington-app · 2026-09-29T15:35:22Z

Sorry, @StanFromIreland and @encukou, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 5c20517a4fc56683efe63a7751020db9573f538d 3.10

bedevere-app · 2026-09-29T15:35:25Z

GH-158429 is a backport of this pull request to the 3.13 branch.

bedevere-app · 2026-09-29T15:35:30Z

GH-158430 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2026-09-29T15:35:43Z

GH-158431 is a backport of this pull request to the 3.15 branch.

bedevere-app · 2026-09-30T10:04:27Z

GH-158489 is a backport of this pull request to the 3.12 branch.

…raryDirectory (GH-157580) (#158489) * gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (cherry picked from commit 5c20517) * Add root user checks to test.support This partially backports commit 86b8617 (GH-146195); making existing tests use the helper is omitted. * gh-134993: Add os.lstat() to os.supports_dir_fd (#135188) Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Victor Stinner <vstinner@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com>

Revert "[3.12] gh-157579: Fix race condition in the cleanup of tempfile.TemporaryDirectory (GH-157580) (#158489)" This reverts commit 458e713.

…oraryDirectory` (GH-157580) (#158431) gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (cherry picked from commit 5c20517) Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com>

…1527) Grype matches CVE-2026-12345 (TemporaryDirectory cleanup race, medium) against the python 3.14.8 binary in the base image and lists a fix only in 3.15.0. The fix is merged on main and 3.15 (python/cpython#157580, #158431); the 3.14 backport (python/cpython#158430) is still open. Refs #1412 Signed-off-by: Jürgen Kreileder <jk@blackdown.de>

…oraryDirectory` (GH-157580) (GH-158430) * gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (cherry picked from commit 5c20517) * Add root user checks to test.support This partially backports commit 86b8617 (GH-146195); making existing tests use the helper is omitted. --------- Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com>

…oraryDirectory` (GH-157580) (#158429) * gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (cherry picked from commit 5c20517) Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com> * Add root user checks to test.support This partially backports commit 86b8617 (GH-146195); making existing tests use the helper is omitted. * Skip test on Android * Skip other affected test --------- Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com> Co-authored-by: T. Wouters <thomas@python.org>

…oraryDirectory` (GH-157580) (GH-158430) (#159074) * [3.12] gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (GH-158430) * gh-157579: Fix race condition in the cleanup of `tempfile.TemporaryDirectory` (GH-157580) (cherry picked from commit 5c20517) * Add root user checks to test.support This partially backports commit 86b8617 (GH-146195); making existing tests use the helper is omitted. --------- (cherry picked from commit 58ed025) Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Petr Viktorin <encukou@gmail.com> * gh-134993: Add os.lstat() to os.supports_dir_fd (#135188) --------- Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Victor Stinner <vstinner@python.org>

hugovk · 2026-10-25T19:51:26Z

Reminder for the 3.11 backport, or remove the label.

….TemporaryDirectory (CVE-2026-12345) When TemporaryDirectory cleanup recovers from a PermissionError it resets permissions and retries removal. It did that by resolving paths again, so an attacker who could modify the tree during cleanup could replace a directory with a symlink and make cleanup delete files, or reset their permissions and flags, outside of the temporary directory. Files are now handled relative to the open directory file descriptors that the fd-based shutil.rmtree() walk already holds. Port of the upstream fix (pythonGH-157580, pythonGH-158430), based on the 3.12 backport (python#159074), plus pythongh-134993 (os.lstat in os.supports_dir_fd). 3.10.22.x: the 3.12 fix relies on 3.12's shutil.rmtree() API (onexc, a public dir_fd parameter, and the _onexc_kwargs switch that passes direntry/dir_fd to the callback). 3.10 only has onerror and a recursive _rmtree_safe_fd(), so this adds the private keyword arguments _dir_fd and _onerror_kwargs to rmtree() instead and threads the parent directory fd and os.DirEntry through _rmtree_safe_fd(). With _onerror_kwargs left false, existing onerror callbacks get the same three positional arguments as before. TemporaryDirectory._rmtree() is ported onto the 3.11 shape of the code (bpo-43153 and pythongh-79325 are picked first), using os.lstat(base, dir_fd=...) where 3.12 uses os.path.isdir()/isjunction() on the full path. Tests are adapted: onerror instead of onexc, local root and platform skips instead of support.requires_non_root_user and PY_SUPPORT_TIER. (cherry picked from commit c39b52d) (cherry picked from commit 5c20517) Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

…TemporaryDirectory (CVE-2026-12345) When TemporaryDirectory cleanup recovers from a PermissionError it resets permissions and retries removal. It did that by resolving paths again, so an attacker who could modify the tree during cleanup could replace a directory with a symlink and make cleanup delete files, or reset their permissions and flags, outside of the temporary directory. Files are now handled relative to the open directory file descriptors that the fd-based shutil.rmtree() walk already holds. Port of the upstream fix (pythonGH-157580, pythonGH-158430), based on the 3.12 backport (python#159074), plus pythongh-134993 (os.lstat in os.supports_dir_fd). 3.10.22.x: the 3.12 fix relies on 3.12's shutil.rmtree() API (onexc, a public dir_fd parameter, and the _onexc_kwargs switch that passes direntry/dir_fd to the callback). 3.10 only has onerror and a recursive _rmtree_safe_fd(), so this adds the private keyword arguments _dir_fd and _onerror_kwargs to rmtree() instead and threads the parent directory fd and os.DirEntry through _rmtree_safe_fd(). With _onerror_kwargs left false, existing onerror callbacks get the same three positional arguments as before. TemporaryDirectory._rmtree() is ported onto the 3.11 shape of the code (bpo-43153 and pythongh-79325 are picked first), using os.lstat(base, dir_fd=...) where 3.12 uses os.path.isdir()/isjunction() on the full path. Tests are adapted: onerror instead of onexc, local root and platform skips instead of support.requires_non_root_user and PY_SUPPORT_TIER. (cherry picked from commit c39b52d) (cherry picked from commit 5c20517) Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> (cherry picked from commit 776b466) 3.9.25.x: tests use test.support instead of test.support.os_helper, and test_parent_mode_preserved only covers the default (no ignore_errors) case because 3.9's TemporaryDirectory has no ignore_cleanup_errors.

StanFromIreland and others added 14 commits August 14, 2026 13:32

Fix tempfile.TemporaryDirectory cleanup race condition

ff74078

Pass descriptors to onexc instead of re-walking

0fd04ae

Petr's review

55c0c1f

Add regression check for _rmtree_use_dir_fd=True

b4c1865

Avoid infinite recursion if directory removal fails

51c9c6a

Bring back chmod by name (avoid regression); use LBYL

bbbb669

Bring back chmod(open(nofollow))

a9a993e

This reduces the attack surface -- readable files avoid the race condition.

Add warning for Windows

f524182

Use lstat; check for it

d4e5045

Test that cleanup() does not chmod the parent directory

4a9eb37

Catch OSError on ignore_errors

4a2ab25

Apply suggestion from @StanFromIreland

99c6f63

Co-authored-by: Stan Ulbrych <stan@python.org>

Match else: to try:, and don't catch AttributeError

ffcf4bb

Use new issue number.

7994121

StanFromIreland requested a review from encukou September 15, 2026 18:12

StanFromIreland requested a review from giampaolo as a code owner September 15, 2026 18:12

bedevere-app Bot mentioned this pull request Sep 15, 2026

Race condition in the cleanup of tempfile.TemporaryDirectory #157579

Open

bedevere-app Bot added the awaiting core review label Sep 15, 2026

Merge branch 'main' into tempfile-toctou-2

10c4541

Add WASI skip

a837624

encukou added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Sep 16, 2026

bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Sep 16, 2026

encukou reviewed Sep 17, 2026

View reviewed changes

Comment thread Lib/test/test_tempfile.py Resolved

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Sep 29, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Sep 29, 2026

bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Sep 29, 2026

StanFromIreland deleted the tempfile-toctou-2 branch September 29, 2026 15:35

zware reviewed Sep 29, 2026

View reviewed changes

Comment thread Misc/NEWS.d/next/Security/2026-08-14-11-55-00.gh-issue-157579.Kq3Vt2.rst Resolved

zware mentioned this pull request Sep 29, 2026

[3.15] gh-157579: Fix race condition in the cleanup of tempfile.TemporaryDirectory (GH-157580) #158431

Merged

This was referenced Sep 29, 2026

lang/python314: fix CVE-2026-12345 (tempfile.TemporaryDirectory race) MidnightBSD/mports#1653

Merged

lang/python313: fix CVE-2026-12345 (tempfile.TemporaryDirectory race) MidnightBSD/mports#1654

Merged

bedevere-app Bot removed the needs backport to 3.12 only security fixes label Sep 30, 2026

Yhg1s added a commit that referenced this pull request Sep 30, 2026

[3.12] GH-157579: Revert GH-158489 (prematurely merged) (#158528)

e1f3590

Revert "[3.12] gh-157579: Fix race condition in the cleanup of tempfile.TemporaryDirectory (GH-157580) (#158489)" This reverts commit 458e713.

This was referenced Oct 4, 2026

ci(grype): ignore CPython tempfile CVE-2026-12345 until a fix ships jkreileder/cf-ips-to-hcloud-fw#1527

Merged

Remove .grype.yaml CVE-2025-15367 ignore (removed in #1556; close once merged) jkreileder/cf-ips-to-hcloud-fw#1412

Closed

StanFromIreland removed the needs backport to 3.10 label Oct 4, 2026

encukou mentioned this pull request Oct 9, 2026

[3.12] gh-157579: Fix race condition in the cleanup of tempfile.TemporaryDirectory (GH-157580) (GH-158430) #159074

Merged

sriaradhyula mentioned this pull request Oct 10, 2026

Track security release verification and remaining vendor advisories caipe-io/ai-platform-engineering#2919

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