Public bug reported:
The genesis of this problem is that the header folding and unfolding
algorithms specified in RFC822 could introduce extraneous white space in
headers which have been folded and unfolded. RFC822 says when folding,
"a CRLF immediately followed by AT LEAST one LWSP-char" may be inserted
where linear-white-space is allowed, and unfolding "is accomplished by
regarding CRLF immediately followed by a LWSP-char as equivalent to the
LWSP-char." Thus, one can insert "AT LEAST one LWSP-char" when folding
but not remove any when unfolding.
The later RFCs 2822 and 5322 clarify the situation by specifying folding
as inserting CRLF preceding existing white-space and unfolding as
removing any CRLF which is immediately followed by white-space.
Various MUAs and supporting libraries including the Python 2 email
library used by Mailman still follow the RFC822 method of folding and
can fold by inserting CRLF followed by TAB and the TAB then doesn't get
removed in unfolding.
There is actually control in the Python email library to use TAB or
SPACE, and Mailman attempts to determine what character has been used
and uses that, but in the case where a Subject: header is not folded but
becomes long enough to be folded because of the insertion of a
subject_prefix for example, Mailman defaults to a TAB which doesn't get
removed in unfolding.
The situation will never be perfect as long as there are MUAs that fold
per RFC822 and MUAs that attempt to compensate by removing some white-
space following CRLF, but it will be improved somewhat by defaulting to
folding with SPACE rather than TAB.
** Affects: mailman
Importance: Low
Assignee: Mark Sapiro (msapiro)
Status: In Progress
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https://bugs.launchpad.net/bugs/1505878
Title:
Mailman can cause extraneous tabs to be displayed in Subject: headers
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