DEBIAN-CVE-2026-116266: Debian/libmime-tools-perl vulnerability

MIME::Head versions from 5.518 before 5.521 for Perl decode a partly encoded multipart boundary without flagging the parse as ambiguous. The multipartboundary() method decodes RFC 2047 encoded words anywhere in the boundary, but the ambiguity check only fires when the whole value is one encoded word. A boundary such as abc=?us-ascii?Q?def?= is used as abcdef, and ambiguouscontent() returns false. Encoded words are not allowed in parameters, so other mail readers split the message on the literal value. A remote sender can hide an attachment from a filter that relies on ambiguouscontent() while the recipient's mail client shows it.