Trailer field names and values written by write_content_chunked()'s
done_with_trailer lambda were never validated, unlike every other
header output path (set_header, WebSocket handshake, client request
headers). An application reflecting untrusted input into a trailer
via DataSink::done_with_trailer() could inject CR/LF sequences and
achieve HTTP response splitting.
Skip trailer fields with invalid names or values, matching the
silent-skip behavior of set_header().
568d434e72
