Unknown severity · Source severity not reportedCVE-2026-116266

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

Answer in brief

CVE-2026-116266 records a Unknown severity vulnerability in MIME::Head versions from 5.518 before 5.521 for Perl decode a partly encoded multipart boundary without flagging the parse as ambiguous. The current sources do not mark it as known exploited. The current feed maps MIME-tools (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 28, 2026Updated Oct 28, 2026Source checked Oct 28, 2026First seen by HOL Oct 28, 2026Material review Oct 28, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps MIME-tools (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
MIME-toolsgeneric	>=5.518 <5.521	5.521

Recommended response

Check inventory. Check lockfiles and deployed manifests for MIME-tools.
Review the reported fix. Source-reported solution: Upgrade to MIME-tools 5.521 or later.

Evidence timeline and material changes

Published upstream
Oct 28, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 28, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 28, 2026

Sources and claim methodology

Source referencemetacpan.org

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.