Bug 2563055 (CVE-2026-116265)

Summary: CVE-2026-116265 perl-MIME-tools: perl-MIME-tools: Security filter bypass via ambiguous multipart boundary continuations
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in perl-MIME-tools. A remote attacker can exploit this vulnerability to bypass email security filters by sending an email with specially crafted multipart boundary parameters. Because the parser does not properly validate boundary parameters split across continuations, it processes message parts differently from standard email clients, allowing hidden attachments to evade inspection while still displaying to the recipient.
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Bug Depends On: 2563408    
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Description OSIDB Bzimport 2026-10-28 17:43:38 UTC 
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 (boundary*0=, boundary*1= 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.