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Medium 2 vendors · 2 advisories

CVE-2026-116266: MIME::Head versions from 5.518 before 5.521 for Perl decode a partly encoded multipart boundary without flagging the parse as ambiguous

CVE-2026-116266 key facts

CISA KEV
Not listed (catalogue checked )
Highest advisory rating
Medium
CVSS base score
5.3 (Red Hat)
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N · base score 5.3, scored by CISA-ADPAttack vector: network. Attack complexity: low. Privileges required: none. User interaction: none. Scope: unchanged. Confidentiality impact: none. Integrity impact: low. Availability impact: none.
Weakness
  • CWE-436 Interpretation Conflict
CISA SSVC
Exploitation: none observed. Automatable: yes. Technical impact: partial. CISA, .
Vendor advisories
2 from Proxmox, Red Hat
Assigned by
CPANSec (CNA)
CVE record published
Listed by VulniPulse
Record changed

Ratings, versions and fixes are quoted from the vendor advisories that cover only CVE-2026-116266; each belongs to its advisory, not to every product listed. How records are collected and checked.

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.

Text from the CVE.org record (CPANSec).

How do I fix CVE-2026-116266?

  • Upgrade to MIME-tools 5.521 or later. (CVE.org record)

Vendor advisories for CVE-2026-116266

Proxmox

1 advisory
  • Advisory severityUnrated

    Advisory [CVE-2026-116264 +9] Proxmox Mail Gateway: Several issues in libmime-tools-perl

    PSA-2026-00091Source published

    This bulletin covers 10 CVEs. The products, versions, score and guidance below describe the bulletin; check its source for applicability to this specific CVE.

    The MIME::Tools package is used for parsing mails in Proxmox Mail Gateway. Recently two new releases for MIME::Tools were released upstream, fixing several issues, including 10, which were assigned CVE identifiers. The issues can result in bypassing certain mail-filter rules similar to issues PSA-2026-00005-1 and PSA-2026-00049-1. Additionally parsing certain messages could result in spending an excessive amount of CPU time by pmg-smtp-filter. PMG used the Max files setting of the Virus detector as general limit on the maximal number of parts in a mail since at least version 5.0. This effectively limited the effect of the unlimited CPU time. The fixes for the issues were backported and are available in version 5.515-1+pmg3 of libmime-tools-perl. Additionally a hard-coded limit on the…

    Affected products in this advisory
    • Proxmox Mail Gateway
    Source-reported affected versions
    Affected-version details not available in this record.
    Source-reported fixed versions
    • libmime-tools-perl 5.515-1+pmg3
    • pmg-api 9.1.5
    Mitigation guidance
    • Update to the fixed package versions: libmime-tools-perl >= 5.515-1+pmg3, pmg-api >= 9.1.5.
    Full advisory and evidenceOpen source bulletin

Red Hat

1 advisory
  • Advisory severityMedium5.3

    Medium [CVE-2026-116266] Security filter bypass via partially encoded multipart boundary

    CVE-2026-116266Source published Source updated

    Security filter bypass via partially encoded multipart boundary. Red Hat rates this moderate (CVSS 5.3). Weakness: CWE-115. Affected products named by the advisory: Red Hat Enterprise Linux 6; Red Hat package: perl-mime-tools.

    Related products — impact not confirmed
    • Red Hat Enterprise Linux 6
    • Red Hat package: perl-mime-tools
    Source-reported affected versions
    • < 5.518
    • < 5.521
    Source-reported fixed versions
    No fixed-version detail extracted. This does not mean no fix exists.
    Mitigation guidance
    No mitigation guidance extracted; consult the source.
    Full advisory and evidenceOpen source bulletin

References

Timeline

  1. CVE record published
  2. First vendor advisory listed by VulniPulse
  3. CVE record last updated

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