New submission from J. Walter Clark <jwaltercl...@gmail.com>:
In various places in the email library `str.splitlines` is used to split up a message where folding might take place in the original message source. This appears to be a bug because when these split parts are re-joined they are joined by a CRLF. https://github.com/python/cpython/blob/ef5bb25e2d6147cd44be9c9b166525fb30485be0/Lib/email/header.py#L369 `str.splitlines` splits on "universal newlines" which can include newlines other than the CRLF. https://docs.python.org/3/library/stdtypes.html#str.splitlines However, the email RFCs define folding whitespace with CRLF as the only possible newline type (optionally surrounded by WSP (SP/HTAB) and/or comments). https://datatracker.ietf.org/doc/html/rfc5322#section-3.2.2 The end result is that a message making a roundtrip through the email parser/generator is mangled because it has any non-CRLF "universal newlines" converted to CRLFs. Anything in the header after the non-CRLF "universal newline" appears on it's own line with no preceding whitespace. This appears to happen with all of the stock policies. ``` from email import message_from_bytes from email.policy import SMTPUTF8 eml_bytes = b'Header-With-FS-Char: BEFORE\x1cAFTER\r\n\r\nBody\r\n' print(eml_bytes) message = message_from_bytes(eml_bytes, policy=SMTPUTF8) print(message.as_bytes(policy=SMTPUTF8)) ``` ``` b'Header-With-FS-Char: BEFORE\x1cAFTER\r\n\r\nBody\r\n' b'Header-With-FS-Char: BEFORE\r\nAFTER\r\n\r\nBody\r\n' ``` The operational impact of this mangling is that the "AFTER" text now makes the message format invalid because it is neither a valid header (no ": ") nor the valid start of a message body (only one CRLF). Common MIME-viewers (e.g. Thunderbird/Outlook) appear to interpret it as a body anyway and any subsequent headers become part of the body. ---------- components: Library (Lib) messages: 411171 nosy: jwalterclark priority: normal severity: normal status: open title: Email Header Folding Converts Non-CRLF Newlines to CRLFs type: behavior versions: Python 3.11 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue46462> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
