---
id: CVE-2026-116265
title: >-
  MIME::Field::ParamVal versions before 5.521 for Perl do not flag an ambiguous
  multipart boundary written as RFC 2231 continuations
summary: >-
  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 22…
severity: none
cwe:
  - CWE-436
product: MIME-tools
affected:
  - MIME-tools < 5.521
published: '2026-10-28'
updated: '2026-10-28'
sourceUpdated: '2026-10-28T17:32:11.827Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-116265'
references:
  - url: 'https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes'
tags:
  - cve.org
ingestedAt: '2026-10-28T18:15:46.384Z'
---

## Overview

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.

## Affected

- `MIME-tools < 5.521`

## Remediation

Upgrade to MIME-tools 5.521 or later.