Limiting the maximum number of tags on Tag.parse_tag()

Would it be worth adding a limit to the maximum number of tags parsed in Tag.parse_tag()?. It builds the cartesian product of compressed tag components:

packaging/src/packaging/tags.py

Lines 232 to 235 in 0030fb8

    
           for interpreter in interpreters.split("."): 
        
               for abi in abis.split("."): 
        
                   for platform_ in platforms.split("."): 
        
                       tags.add(Tag(interpreter, abi, platform_))

The spec does warn about it, but doesn't actually specify a limit for them

This compression scheme can generate large numbers of unsupported tags and “impossible” tags that are supported by no Python implementation e.g. “cp33-cp31u-win64”, so use it sparingly.

Are you worried about some attack that will exhaust memory?

Are you worried about some attack that will exhaust memory?

Not realistically, if we assume we're parsing a maliciously crafted wheel, it'd be limited by the filesystem limits on filename length (usually 255 bytes). You could craft a filename such that it expands to about ~40k tags:

{name}-{version}-{python}-{abi}-{platform}.whl

a-1-<36 parts>-<36 parts>-<36 parts>.whl

a-1-a.b.c....-a.b.c...-a.b.c....whl

36 * 36 * 36 = 46.656

which shouldn't be huge memory-wise.

I opened the issue to ask if it makes sense to limit it more from the "there's no good reason to allow it" side than the "this is definitely exploitable" side.

Mm thinking about it further, it could be that parse_tag() is called with a tag extracted from a string that does not correspond to an actual file in a filesystem (like the URL for a wheel inside a requirements.txt file). So in that case there's no upper limit for the length of the filename.

Ultimately this isn't an issue for packaging, it's callers of packaging processing untrusted data. They would be best processing such data in some kind of time limited and memory limited sandbox.

To help such users out I think we should add a key word only argument like "maximum_tags”, with either a default of None (unbounded) or some sufficiently large number that is likely to meet all realistic use cases.

To help such users out I think we should add a key word only argument like "maximum_tags”, with either a default of None (unbounded) or some sufficiently large number that is likely to meet all realistic use cases.

I was thinking of something similar: a limit keyword-only argument that defaults to None and if set would raise an exception if the limit is hit before completing.

Makes sense to me, I created a PR for it here: #1224

Do we call parse_tag under the hood in any of our other objects or helper utilities?

It may change how we need to expose the limit option.

@facutuesca

I know I'm commenting on an already closed issue, but I think a reasonable limit here might protect many application that uses this. For example pip calls parse_wheel_filename in their Wheel.__init__ and there is currently no way to use this limit for protection. And as @facutuesca pointed out, we are not limited to what filesystems allow us because a crafted requirements.txt file might cause pip to consume hundreds of MBs of memory even before it tries to touch an actual file.

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Limiting the maximum number of tags on `Tag.parse_tag()` #1220

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	for interpreter in interpreters.split("."):
	for abi in abis.split("."):
	for platform_ in platforms.split("."):
	tags.add(Tag(interpreter, abi, platform_))

Limiting the maximum number of tags on Tag.parse_tag() #1220

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brettcannon commented on Jun 3, 2026

facutuesca commented on Jun 3, 2026

facutuesca commented on Jun 3, 2026

notatallshaw commented on Jun 3, 2026

brettcannon commented on Jun 4, 2026

facutuesca commented on Jun 4, 2026

notatallshaw commented on Jun 11, 2026

frenzymadness commented on Jul 28, 2026

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Limiting the maximum number of tags on `Tag.parse_tag()` #1220