New submission from J. Walter Clark :
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
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<https://bugs.python.org/issue46462>
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