CVE-2026-116265 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 do not flag an ambiguous multipart boundary written as RFC 2231 continuations.

The boundary ambiguity checks look only at a plain boundary= parameter, and a boundary split into RFC 2231 sections (boundary0=, boundary1= and so on) is joined after they have run. A repeated section number is concatenated rather than reported as a duplicate. From version 5.518, an encoded word or trailing whitespace in such a boundary also goes unflagged, although multipart_boundary() decodes or strips it.

A remote sender can make MIME::Parser split a message on a different boundary than other mail readers, hiding 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-2026-116265
• https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes