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Fix max_incomplete_event_size for complete oversized headers - #206
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Previously the size limit was only checked when next_event() returned NEED_DATA, so oversized request/response headers that arrived in a single chunk (or otherwise completed without an incomplete overflow) were accepted while the same bytes split across packets were rejected. Enforce the limit using bytes consumed by header events as well (python-hyper#182).
Wkhanex
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Oct 16, 2026
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I checked 05972a8b34fa on both the server request path and client response path, at every byte split point and at limits one byte below, equal to, and one byte above the serialized header length. Over-limit cases consistently raised RemoteProtocolError with status hint 431; exact-limit headers remained accepted even when followed in the same receive call by a 400-byte body.
All 1,424 additional assertions and the full project suite (78 tests) passed on CPython 3.12. The coalesced body control is particularly useful for the before - len(buffer) calculation: it confirms body bytes do not count toward the header limit. AI-assisted validation at the pinned head.
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Summary
max_incomplete_event_sizewhen a complete header event is parsed, not only onNEED_DATA.Closes / relates to #182.
Test plan
pytest h11/tests/test_connection.py::test_max_incomplete_event_size_countermeasure