---
id: "CVE-2026-116266"
kind: "vulnerability"
aliases: ["CVE-2026-116266"]
title: "Perl MIME::Head: partly encoded boundary hides attachments from mail filters"
severity: "medium"
cvss: null
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["email-security", "mail-filtering", "package", "perl"]
not_affected_if: ["no-mime-head-usage"]
published_at: "2026-10-28T18:17:13+00:00"
first_seen_at: "2026-10-28T18:34:33+00:00"
updated_at: "2026-10-28T18:35:30+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-116266", "first_seen_at": "2026-10-28T18:34:33+00:00"}]
---

# Perl MIME::Head: partly encoded boundary hides attachments from mail filters

MIME::Head for Perl decodes RFC 2047 encoded words inside a multipart boundary but skips the ambiguity check unless the whole boundary is one encoded word. A crafted email can therefore slip an attachment past a filter that relies on ambiguous_content() while the recipient's mail client still displays it.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed MIME::Head version with `perl -MMIME::Head -e 'print $MIME::Head::VERSION'`
- Find where the code calls `multipart_boundary()` and `ambiguous_content()` to see whether mail filtering depends on them

## Fix

- Upgrade MIME::Head to `5.521` or later

## Timeline

- 2026-10-28T18:34:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-116266

## From the sources

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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_