[3.14] gh-90949: expose Expat API to tune exponential expansion protections (GH-139368) by StanFromIreland · Pull Request #150496 · python/cpython

StanFromIreland · 2026-05-26T20:44:31Z

Expose the XML Expat 2.7.2 APIs to tune protections against "billion laughs" 1 attacks.

The exposed APIs are available on Expat parsers, that is, parsers created by xml.parsers.expat.ParserCreate(), as:

parser.SetBillionLaughsAttackProtectionActivationThreshold(threshold), and
parser.SetBillionLaughsAttackProtectionMaximumAmplification(max_factor).

This completes the work in f04bea4, and improves the existing related documentation.

Issue: Expose Expat XML attack protection APIs #90949

… protections (pythonGH-139368) Expose the XML Expat 2.7.2 APIs to tune protections against "billion laughs" [1] attacks. The exposed APIs are available on Expat parsers, that is, parsers created by `xml.parsers.expat.ParserCreate()`, as: - `parser.SetBillionLaughsAttackProtectionActivationThreshold(threshold)`, and - `parser.SetBillionLaughsAttackProtectionMaximumAmplification(max_factor)`. This completes the work in f04bea4, and improves the existing related documentation. [1]: https://en.wikipedia.org/wiki/Billion_laughs_attack (cherry picked from commit 6661123) Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

read-the-docs-community · 2026-05-26T20:51:48Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #33057152 | 📁 Comparing a35a24a against main (8ab7b43)

🔍 Preview build

393 files changed · ± 376 modified · - 17 deleted

± Modified

- Deleted

picnixz · 2026-05-26T21:10:38Z

        self.assert_root_parser_failure(setter, 123.45)


+@unittest.skipIf(expat.version_info < (2, 4, 0), "requires Expat >= 2.4.0")


I think we had a recent issue where this check was not sufficient and tests needed to be disabled in another way using hasattr() checks. Can you check how it's currently done on main please?

It's identical on main?

cpython/Lib/test/test_pyexpat.py

Lines 1265 to 1266 in d8ff4f8

@unittest.skipIf(expat.version_info < (2, 4, 0), "requires Expat >= 2.4.0")

class ExpansionProtectionTest(AttackProtectionTestBase, unittest.TestCase):

Ah indeed, but the problem was for the other API:

@unittest.skipIf(not hasattr(expat.XMLParserType, "SetAllocTrackerMaximumAmplification"), "requires Python compiled with Expat >= 2.7.2")

So I think the version check for billion laughs is also wrong and should be changed to the same as for SetAllocTrackerMaximumAmplification. But let's address this separately later in a follow-up PR (so as to ease backports)

picnixz · 2026-05-27T15:42:14Z

LGTM but I'd appreciate if @hartwork could have a quick glance just for wordings. I hope every backport has been merged properly (if we are matching against 3.15 then it should good). It's just that I remember that the work was split across multiple PRs...

hartwork · 2026-05-28T09:55:56Z

LGTM but I'd appreciate if @hartwork could have a quick glance just for wordings. I hope every backport has been merged properly (if we are matching against 3.15 then it should good). It's just that I remember that the work was split across multiple PRs...

@StanFromIreland I have just had a quick look through the lense of…

# diff -u1 <(git diff 666112376d574c7802646ee1df6244062671cd61{^,}) <(git diff upstream-readonly/3.14...StanFromIreland/backport-6661123-3.14) | ydiff

…and I see:

differences in @unittest.skipIf machinery
differences in float style "100.0" versus int style "100" in documentation

Are these intended or accidental?

picnixz · 2026-05-28T10:12:09Z

differences in float style "100.0" versus int style "100" in documentation

I think I updated these at some points but I don't remember in which direction. I would say: pick the style that we currently have on main, it doesn't matter. I just don't want to have docs that are different across versions.

hartwork · 2026-05-28T10:17:21Z

@picnixz +1 from me for being in line with main 👍

picnixz · 2026-06-09T11:51:25Z

+    capi->SetAllocTrackerActivationThreshold = NULL;
+    capi->SetAllocTrackerMaximumAmplification = NULL;


the else is the wrong here actually, it's SetBillionLaughsAttackProtectionActivationThreshold

It's the same on main:

cpython/Modules/pyexpat.c

Lines 2548 to 2554 in 9fdbade

#if XML_COMBINED_VERSION >= 20400

capi->SetBillionLaughsAttackProtectionActivationThreshold = XML_SetBillionLaughsAttackProtectionActivationThreshold;

capi->SetBillionLaughsAttackProtectionMaximumAmplification = XML_SetBillionLaughsAttackProtectionMaximumAmplification;

#else

capi->SetAllocTrackerActivationThreshold = NULL;

capi->SetAllocTrackerMaximumAmplification = NULL;

#endif

I wrote #151147 for main/3.15, for here:

Suggested change

capi->SetAllocTrackerActivationThreshold = NULL;

capi->SetAllocTrackerMaximumAmplification = NULL;

capi->SetBillionLaughsAttackProtectionActivationThreshold = NULL;

capi->SetBillionLaughsAttackProtectionMaximumAmplification = NULL;

picnixz

Now I think we're good. If docs need to be amended we'll do it but C should be ok.

StanFromIreland · 2026-06-09T12:41:47Z

Fuzzer failures are unrelated (ast and plistlib).

miss-islington-app · 2026-06-09T12:42:36Z

Thanks @StanFromIreland for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

miss-islington-app · 2026-06-09T12:42:40Z

Sorry, @StanFromIreland, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c078637e8642783d90b33c6b983fa15ff29c24d6 3.13

bedevere-app · 2026-06-09T12:47:55Z

GH-151151 is a backport of this pull request to the 3.13 branch.

StanFromIreland requested a review from picnixz May 26, 2026 20:44

This was referenced May 26, 2026

Expose Expat XML attack protection APIs #90949

Open

gh-90949: expose Expat API to tune exponential expansion protections #139368

Merged

bedevere-app Bot added the awaiting core review label May 26, 2026

picnixz reviewed May 26, 2026

View reviewed changes

Comment thread Modules/pyexpat.c Outdated

picnixz reviewed May 26, 2026

View reviewed changes

Comment thread Modules/pyexpat.c Outdated

StanFromIreland marked this pull request as draft May 26, 2026 21:49

bedevere-app Bot removed the awaiting core review label May 26, 2026

Fix some extraneous whitespace

1493ed4

StanFromIreland marked this pull request as ready for review May 27, 2026 15:20

bedevere-app Bot added the awaiting core review label May 27, 2026

StanFromIreland requested a review from picnixz May 27, 2026 15:20

coderabbitai Bot mentioned this pull request Jun 3, 2026

feat: add docx redline script with style tracking arthrod/html-to-docx2#470

Open

picnixz reviewed Jun 9, 2026

View reviewed changes

Comment thread Modules/pyexpat.c Outdated

Whoops, re-indent after a previous un-indent

6f53983

StanFromIreland requested a review from picnixz June 9, 2026 11:49

picnixz reviewed Jun 9, 2026

View reviewed changes

Apply suggestion from @StanFromIreland

a35a24a

StanFromIreland requested a review from picnixz June 9, 2026 12:04

picnixz approved these changes Jun 9, 2026

View reviewed changes

bedevere-app Bot added awaiting merge and removed awaiting core review labels Jun 9, 2026

StanFromIreland merged commit c078637 into python:3.14 Jun 9, 2026
52 of 55 checks passed

bedevere-app Bot removed the awaiting merge label Jun 9, 2026

StanFromIreland deleted the backport-6661123-3.14 branch June 9, 2026 12:42

StanFromIreland added the needs backport to 3.13 bugs and security fixes label Jun 9, 2026

miss-islington-app Bot assigned StanFromIreland Jun 9, 2026

hugovk mentioned this pull request Jun 9, 2026

[3.14] gh-149018: Use XML_SetHashSalt16Bytes in pyexpat/_elementtree when possible (GH-149023) #149646

Merged

StanFromIreland removed the needs backport to 3.13 bugs and security fixes label Jun 9, 2026

		self.assert_root_parser_failure(setter, 123.45)


		@unittest.skipIf(expat.version_info < (2, 4, 0), "requires Expat >= 2.4.0")

	@unittest.skipIf(expat.version_info < (2, 4, 0), "requires Expat >= 2.4.0")
	class ExpansionProtectionTest(AttackProtectionTestBase, unittest.TestCase):

		capi->SetAllocTrackerActivationThreshold = NULL;
		capi->SetAllocTrackerMaximumAmplification = NULL;

	#if XML_COMBINED_VERSION >= 20400
	capi->SetBillionLaughsAttackProtectionActivationThreshold = XML_SetBillionLaughsAttackProtectionActivationThreshold;
	capi->SetBillionLaughsAttackProtectionMaximumAmplification = XML_SetBillionLaughsAttackProtectionMaximumAmplification;
	#else
	capi->SetAllocTrackerActivationThreshold = NULL;
	capi->SetAllocTrackerMaximumAmplification = NULL;
	#endif

Uh oh!

Conversation

StanFromIreland commented May 26, 2026 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

read-the-docs-community Bot commented May 26, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Documentation build overview

Uh oh!

picnixz May 26, 2026

Choose a reason for hiding this comment

Uh oh!

StanFromIreland May 27, 2026

Choose a reason for hiding this comment

Uh oh!

picnixz May 27, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

picnixz commented May 27, 2026

Uh oh!

hartwork commented May 28, 2026

Uh oh!

picnixz commented May 28, 2026

Uh oh!

hartwork commented May 28, 2026

Uh oh!

Uh oh!

picnixz Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

StanFromIreland Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

StanFromIreland Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

picnixz Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

picnixz left a comment

Choose a reason for hiding this comment

Uh oh!

StanFromIreland commented Jun 9, 2026

Uh oh!

Uh oh!

miss-islington-app Bot commented Jun 9, 2026

Uh oh!

miss-islington-app Bot commented Jun 9, 2026

Uh oh!

bedevere-app Bot commented Jun 9, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

StanFromIreland commented May 26, 2026 •

edited by bedevere-app Bot

Loading

read-the-docs-community Bot commented May 26, 2026 •

edited

Loading