parse_mimetype: skip empty-key parameters and segments without '='#13052
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HrachShah wants to merge 3 commits into
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parse_mimetype: skip empty-key parameters and segments without '='#13052HrachShah wants to merge 3 commits into
HrachShah wants to merge 3 commits into
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July 4, 2026 19:27
The MIME parameter parser in parse_mimetype treated every segment after the
type/subtype as a valid parameter, even when the segment was malformed. Three
real-world patterns were silently producing the spurious empty-key entry
{'': 'value'} in the parsed parameters dict, which downstream callers like
StringPayload read with .get('charset') and silently fall back to utf-8
without warning:
* 'text/html;' (bare trailing ';') was already skipped via the existing
'if not item: continue' check, but a whitespace-only segment slipped
through because ''.strip() is truthy in Python.
* 'text/html;charset=utf-8' (no space after ';') was treated as a
parameter named 'charset=utf-8' with an empty value, not as a
charset=utf-8 parameter.
* 'text/html;=value' (empty key, present '=') was treated as a parameter
with an empty key.
Per RFC 9110 section 5.6.5, a MIME type parameter is 'token = token' (or
quoted-string), so both the '=' and a non-empty key are required for a
segment to be a valid parameter. The fix:
* replace the unconditional partition() with a check for the '=' separator
(str.partition returns '' as the separator when the needle is absent);
* strip the key and skip segments whose key is empty after stripping;
* drop the now-redundant outer 'if not item: continue' check (a bare
';' is also a 'no =' segment and is caught by the new check).
The pre-existing 'text/plain;base64' case in the parametrize fixture, which
the old code parsed as {'base64': ''}, is removed: per the new behaviour
';base64' is a malformed segment (no '=') and is skipped, so the parsed
parameters are {}. The new behavior matches RFC 9110 and matches the
existing pre-fix behavior for a bare trailing ';'.
New test test_parse_mimetype_skips_empty_param_key_and_missing_equals
covers the trailing-';' case and asserts that the real 'charset' parameter
is still parsed when it precedes the malformed trailing segment, so the
fix does not regress the happy path.
Tests:
python3 -m pytest tests/test_helpers.py -k parse_mimetype
=> 9 passed (8 pre-existing + 1 new)
python3 -m pytest tests/test_helpers.py
=> 1 pre-existing failure (re_chunked_parse) and the rest pass
This was referenced Jul 4, 2026
for more information, see https://pre-commit.ci
Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## master #13052 +/- ##
==========================================
- Coverage 98.96% 98.95% -0.01%
==========================================
Files 131 131
Lines 48156 48165 +9
Branches 2499 2501 +2
==========================================
+ Hits 47656 47661 +5
- Misses 376 378 +2
- Partials 124 126 +2
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The MIME parameter parser in
parse_mimetypetreated every segment after the type/subtype as a valid parameter, even when the segment was malformed. Three real-world patterns were silently producing the spurious empty-key entry{'': 'value'}in the parsed parameters dict, which downstream callers likeStringPayloadread with.get('charset')and silently fall back to utf-8 without warning:text/html;(whitespace-only segment after;) was treated as a parameter named''with value''. The pre-existingif not item: continueonly skipped a bare trailing;.text/html;charset=utf-8(no space after;) was treated as a parameter named'charset=utf-8'with an empty value, not as acharset=utf-8parameter.text/html;=value(empty key, present=) was treated as a parameter named''with value'value'.text/html;charset(no=at all) was treated as a parameter named'charset'with an empty value, instead of being skipped as malformed.The fix in
aiohttp/helpers.py:partition("=")and checks the separator explicitly. A missing=causes the segment to be skipped, matching the existing behaviour for the empty-segment case.;=valueor; =valuesegment does not produce a{'': 'value'}entry.Tests in
tests/test_helpers.py:text/plain;base64parametrized case (which relied on the old behaviour of producing{'base64': ''}) is removed.text/plain;base64is a malformed MIME header per RFC 9110; the new behaviour is to skip it.test_parse_mimetype_skips_empty_param_key_and_missing_equalscovers the;charset=utf-8;(trailing-;after a real param) case to make sure the real parameter is still parsed.Verification:
python3 -m pytest tests/test_helpers.py -k parse_mimetypepasses 9/9.; charset=utf-8;segment inserts''into parameters, and the pre-existing test still expects{'base64': ''}for thetext/plain;base64case).Refs: #13009 (whitespace-only segments), #13002 (OWS before semicolon, related).