# CVE-2026-63435

## Summary

- **CVE ID:** CVE-2026-63435
- **Severity:** MEDIUM
- **CVSS Score:** 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)
- **CWE:** CWE-436
- **Published:** Sep 1, 2026
- **Last Modified:** Sep 2, 2026

## 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.

## Affected Products

- mikel — mail (< 2.9.1)

## References

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

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.33%
- **EPSS Percentile:** 25.5

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-09._