CVE-2026-63435

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.

• CWE-436

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

Published

Sep 1, 2026

Last Modified

Sep 9, 2026

Status

Deferred

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