Medium [CVE-2026-116266] Security filter bypass via partially encoded multipart boundary
This medium-severity Red Hat Linux advisory covers CVE-2026-116266 affecting Red Hat Enterprise Linux 6, Red Hat package: perl-mime-tools.
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Summary
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. A flaw was found in perl-mime-tools.
When parsing email headers, the library decodes partially encoded boundary values in multipart messages without flagging the content as ambiguous.
A remote attacker can exploit this flaw by sending a specially crafted email message to bypass email security filters that inspect attachments, while still allowing the recipient's mail client to display the content. Red Hat severity: Moderate — CVSS 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N).
Weakness: CWE-115. Affected Red Hat products: Red Hat Enterprise Linux 6.
Will not fix / out of support: Red Hat Enterprise Linux 6. Red Hat does not currently list a fixing RHSA for this CVE.
- < 5.518
- < 5.521
Official advisory · high-confidence parse· fetched 9 hours ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists. Confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 9 hours ago·verify at source
Mitigation
The source record does not include mitigation steps. That does not mean no fix exists. Read the vendor advisory below for the authoritative guidance.
Official advisory · high-confidence parse· fetched 9 hours ago·verify at source
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