When folding a long comment in an email header containing exclusively unfoldable characters, the parenthesis would not be preserved. This could be used for injecting headers into email messages where addresses are user-controlled and not sanitized.
References
- https://github.com/python/cpython/commit/17d1490aa97bd6b98a42b1a9b324ead84e7fd8a2
- https://github.com/python/cpython/commit/61614a5e5056e4f61ced65008d4576f3df34acb6
- https://github.com/python/cpython/commit/e9970f077240c7c670e8a6fc6662f2b30d3b6ad0
- https://github.com/python/cpython/commit/f738386838021c762efea6c9802c82de65e87796
- https://github.com/python/cpython/issues/143935
- https://github.com/python/cpython/pull/143936
- https://mail.python.org/archives/list/security-announce@python.org/thread/FELSEOLBI2QR6YLG6Q7VYF7FWSGQTKLI/