Tighten the parsing logic of chunked encoding #78
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According to RFC 9112, a chunk size consists of a series of hexadecimal characters, which may be optionally followed by whitespace and extensions beginning with a semicolon.
Picohttpparser's parsing logic has been more permissive. It recognizes a sequence of hexadecimal characters as the size, disregarding any subsequent characters. For example, although "123x\r\n" does not conform to RFC 9112, picohttpparser has interpreted this line as indicating a chunk size of 291 (0x123) bytes.
Recently, concerns have surfaced about this lenient behavior. Specifically, there is a risk that if a client, acting as a proxy, forwards a malformed chunk size which is interpreted differently by that client, it could enable splitting attacks.
To mitigate the concern, this PR tightens the parsing logic. Now, the first non-hexadecimal character must be a whitespace, a semicolon, or the end of line. While it is the responsibility of each endpoint to send HTTP messages in accordance with the specifications, these changes ensure a more robust handling of potential deviations.
We express our gratitude to Ben Kallus and Keran Mu for independently bringing this issue to our attention.