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CVE-2026-116266                                      CPAN Security Group
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         CVE ID:  CVE-2026-116266

   Distribution:  MIME-tools
       Versions:  from 5.518 before 5.521
       MetaCPAN:  https://metacpan.org/dist/MIME-tools
       VCS Repo:  https://salsa.debian.org/dskoll/mime-tools

MIME::Head versions from 5.518 before 5.521 for Perl decode a partly
encoded multipart boundary without flagging the parse as ambiguous

Description
-----------
MIME::Head versions from 5.518 before 5.521 for Perl decode a partly
encoded multipart boundary without flagging the parse as ambiguous.

The multipart_boundary() 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 ambiguous_content() 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 ambiguous_content() while the
recipient's mail client shows it.

Problem types
-------------
- CWE-436 Interpretation Conflict

Solutions
---------
Upgrade to MIME-tools 5.521 or later.

References
----------
https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes