Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.web.http.HTTPChannel.headerReceived rejects a NUL byte in an HTTP header value but accepts a bare carriage return or bare line feed. Because Twisted splits header lines only on CRLF, a hidden Content-Length header can remain folded into another value while a lenient front end treats the bare character as a line terminator. The resulting framing disagreement can smuggle a following request past front-end access controls, poison a shared connection or cache, and expose a later client to the wrong response. This issue is an incomplete-fix residual of earlier Twisted request-smuggling protections. This issue is fixed in version 26.10.0rc1.

Affected: PyPI: Twisted

## Summary

An incomplete header-value validation (CWE-444) in the Twisted HTTP server lets a bare carriage return or line feed in a header value hide a following header from the parser. `twisted.web.http.HTTPChannel.headerReceived` rejects a NUL byte in a header value but not a bare CR or LF, even though RFC 9110 section 5.5 requires a recipient to reject all three. Because the channel splits requests only on CRLF, a value carrying a bare LF and a second header arrives as one line, so the second header is folded into the value and the request is framed with no body. A front-end that treats the bare LF as a line terminator frames the request differently, and the disagreement lets an attacker smuggle a request past it. This is an incomplete fix of the earlier Twisted request-smuggling advisories, which guarded the NUL sibling of this exact check.

## Details

`headerReceived` splits a header line on the first colon, strips space and tab from the value, and validates it with a single test that rejects only NUL:

```python # twisted/web/http.py HTTPChannel.headerReceived data = data.strip(b" \t") if b"\x00" in data: # NUL rejected; a bare CR or LF is not self._respondToBadRequestAndDisconnect() return False ```

The channel delimiter is CRLF, so `Foo: x\nContent-Length: 39` is one line: the split yields name `Foo` and value `x\nContent-Length: 39`, `strip` leaves the LF, and the NUL check passes. `_maybeChooseTransferDecoder` is then called with `Foo`, never s…