fix(body-limit): refuse chunked & malformed Content-Length (F3, F21)

[jieyao-MilestoneHub]

Summary

BodySizeLimitMiddleware only enforced the cap when a Content-Length header was present and parsed as a non-negative int. Two bypasses:

• F3 — chunked transfer-encoding bypass. A POST with Transfer-Encoding: chunked and no Content-Length flowed straight through. Starlette buffers the entire chunked body into memory before pydantic sees it, so the cap was ineffective for any client that chose chunked. Honest JSON clients (httpx, openai-python, langchain-openai, curl) always set Content-Length on JSON POSTs, so the practical impact of refusing chunked is zero. Now: body-bearing methods (POST / PUT / PATCH) without Content-Length are refused with 411 Length Required. Operators who genuinely need chunked uploads must front the gateway with a proxy that materialises Content-Length, or extend this middleware to a streaming ASGI implementation.
• F21 — malformed Content-Length silently zero-ed. Old code did try: int(raw); except: declared = 0. A hostile client sending Content-Length: abc got a free pass — the body was unbounded, the cap-comparison saw 0. Now: malformed values return 400 Bad Request. Negative values also return 400.

Module docstring updated to reflect the stricter contract.

Behaviour change

Request shape	Before	After
Content-Length: 100, body 100 B, cap 1 KiB	200 ✓	200 ✓
Content-Length: 9999, cap 1 KiB	413 ✓	413 ✓
Content-Length: abc, body anything	silently passes	400
Content-Length: -1, body anything	silently passes	400
Transfer-Encoding: chunked POST	silently passes	411
GET /anything, no Content-Length	passes ✓	passes ✓
GET /health, no Content-Length	passes ✓	passes ✓

Test plan

• Updated existing test_malformed_content_length_header_treated_as_zero (which asserted the old silent-pass behaviour) → now test_malformed_content_length_header_returns_400
• New test_negative_content_length_returns_400
• CI: existing POST happy-path / GET-bypass / oversized tests continue to pass
• CI: ruff + black + pyright pass

Related

Findings F3 (chunked bypass), F21 (malformed CL silently zero-ed) from internal security review punch list.

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