CVE-2026-116264 Affecting libmime-tools-perl package, versions <5.522-1

Note: Versions mentioned in the description apply only to the upstream libmime-tools-perl package and not the libmime-tools-perl package as distributed by Debian. See How to fix? for Debian:unstable relevant fixed versions and status.

MIME::Field::ParamVal versions before 5.521 for Perl allow one invalid Content-Type parameter to hide the parameters after it, including the multipart boundary.

Parameter parsing stops at the first parameter whose name it cannot read, such as x:y=1 or a name containing an 8-bit byte, and drops every parameter after it. When the boundary is among them, MIME::Parser keeps the multipart body as a single undecoded part without marking the parse as ambiguous.

Mail readers that skip the invalid parameter still find the boundary. A remote sender can hide an attachment from a filter that relies on ambiguous_content() while the recipient's mail client shows it.

• https://security-tracker.debian.org/tracker/CVE-2026-116264
• https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes