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

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>

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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)

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...

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?

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.

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

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;

Oh!

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

Fuzzer failures are unrelated (ast and plistlib).

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

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

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

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

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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

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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

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Whoops, re-indent after a previous un-indent

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

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Apply suggestion from @StanFromIreland

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

picnixz approved these changes Jun 9, 2026

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StanFromIreland merged commit c078637 into python:3.14 Jun 9, 2026
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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

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hugovk mentioned this pull request Jun 9, 2026

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

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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

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

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[3.14] gh-90949: expose Expat API to tune exponential expansion protections (GH-139368) - #150496

[3.14] gh-90949: expose Expat API to tune exponential expansion protections (GH-139368)#150496StanFromIreland merged 4 commits intopython:3.14python/cpython:3.14from StanFromIreland:backport-6661123-3.14StanFromIreland/cpython:backport-6661123-3.14Copy head branch name to clipboard

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[3.14] gh-90949: expose Expat API to tune exponential expansion protections (GH-139368)#150496
StanFromIreland merged 4 commits into
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