actix/actix-web · error · io::Error

Invalid chunk size line: Invalid Size

Error message

Invalid chunk size line: Invalid Size

What it means

Raised in ChunkedState::read_size (chunked.rs:65) when the first byte of a chunk-size field is not a hex digit and not one of the allowed delimiters (space/tab, ';', or CR). It signals a structurally invalid chunked transfer-encoding body and surfaces upstream as a PayloadError::Io, typically causing a 400 Bad Request when reading the request body.

Solutions

  1. Ensure the client emits chunk sizes as lowercase/uppercase hexadecimal followed by CRLF.
  2. If behind a reverse proxy, make proxy and backend agree: either the proxy fully de-chunks and sends Content-Length, or it forwards valid chunked framing.
  3. Handle the resulting PayloadError and respond 400 rather than 500.

Example fix

// before (client body)
"G\r\nhello\r\n0\r\n\r\n"
// after
"5\r\nhello\r\n0\r\n\r\n"
Defensive patterns

Strategy: try-catch

Try / catch

// Errors surface while reading the body payload
while let Some(res) = payload.next().await {
    match res {
        Ok(chunk) => { /* process */ }
        Err(PayloadError::Io(e)) if e.kind() == io::ErrorKind::InvalidInput =>
            return HttpResponse::BadRequest().finish(),
        Err(e) => return Err(e.into()),
    }
}

Prevention

When it happens

Trigger: A chunked request body begins a chunk-size line with a non-hex character, e.g. the body sent "G\r\n..." or "x4\r\ndata\r\n". The byte! macro reads one byte and the wildcard arm at chunked.rs:64 fires.

Common situations: A buggy HTTP client not emitting hex chunk sizes; a request smuggling attempt; a proxy that strips or corrupts the chunked encoding; misconfigured reverse proxy forwarding a de-chunked body but leaving Transfer-Encoding: chunked.

Related errors

AI-assisted analysis of actix/actix-web@4d435abc28 (2026-08-09). Data as JSON: /api/errors/54f609f9dcd01e72. Report an issue: GitHub.

Appendix: source

Thrown at actix-http/src/h1/chunked.rs:65

            BodyLf => ChunkedState::read_body_lf(body),
            EndCr => ChunkedState::read_end_cr(body),
            EndLf => ChunkedState::read_end_lf(body),
            End => Poll::Ready(Ok(ChunkedState::End)),
        }
    }

    fn read_size(rdr: &mut BytesMut, size: &mut u64) -> Poll<Result<ChunkedState, io::Error>> {
        let radix = 16;

        let rem = match byte!(rdr) {
            b @ b'0'..=b'9' => b - b'0',
            b @ b'a'..=b'f' => b + 10 - b'a',
            b @ b'A'..=b'F' => b + 10 - b'A',
            b'\t' | b' ' => return Poll::Ready(Ok(ChunkedState::SizeLws)),
            b';' => return Poll::Ready(Ok(ChunkedState::Extension)),
            b'\r' => return Poll::Ready(Ok(ChunkedState::SizeLf)),
            _ => {
                return Poll::Ready(Err(io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "Invalid chunk size line: Invalid Size",
                )));
            }
        };

        match size.checked_mul(radix) {
            Some(n) => {
                *size = n;
                *size += rem as u64;

                Poll::Ready(Ok(ChunkedState::Size))
            }
            None => {
                debug!("chunk size would overflow u64");
                Poll::Ready(Err(io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "Invalid chunk size line: Size is too big",

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