Mishandling of comma during folding and unicode-encoding of email headers

Vulnerability Description

During an address list folding when a separating comma ends up on a folded line and that line is to be unicode-encoded then the separator itself is also unicode-encoded. Expected behavior is that the separating comma remains a plan comma. This can result in the address header being misinterpreted by some mail servers.

References

• https://github.com/python/cpython/issues/100884
• https://github.com/python/cpython/pull/100885
• https://github.com/python/cpython/pull/119099
• https://github.com/python/cpython/commit/09fab93c3d857496c0bd162797fab816c311ee48
• https://github.com/python/cpython/commit/70754d21c288535e86070ca7a6e90dcb670b8593
• https://github.com/python/cpython/commit/9148b77e0af91cdacaa7fe3dfac09635c3fe9a74
• https://mail.python.org/archives/list/security-announce@python.org/thread/MB62IZMEC3UM6SGHP5LET5JX2Y7H4ZUR/
• https://github.com/python/cpython/commit/a4ef689ce670684ec132204b1cd03720c8e0a03d
• https://github.com/python/cpython/commit/d4df3c55e4c5513947f907f24766b34d2ae8c090