GHSA-mvxr-6m87-mv2q

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GHSA-mvxr-6m87-mv2q

Summary: Mail: Email address spoofing via malformed RFC 2047 encoded-words

Details: ## Summary Mail::Utilities.q_value_decode and Mail::Utilities.b_value_decode decoded only the first RFC 2047 encoded-word in a string and used an overly greedy pattern to match the charset token. A crafted, malformed encoded-word embedded in an address display name or local part could cause the decoded output to differ from what a human reviewer or downstream parser would expect, allowing an attacker to spoof the apparent sender/recipient address. ## Details Both decoders used a single String#match against a pattern such as /\=\?(.+)?\?[Qq]\?(.*)\?\=/m. Two problems: 1. Single match, dropped remainder. Only the first =?charset?Q?...?= (or ?B?) word was decoded. Any additional encoded-words or surrounding text were not handled consistently, so the decoded result could silently omit or alter parts of the input. 2. Greedy charset capture. (.+)? is greedy and matches across ? delimiters, so a malformed word could span more of the string than a strict RFC 2047 parse would, changing the boundary between "encoded" and "literal" text. ## Impact Applications using mail to parse and display or authorize based on decoded header values (From, To, Reply-To, etc.) may present or act on an address different from the one a validator inspecting the raw header would see. Primary risk is spoofing / phishing and authorization-check bypass. No RCE.

References: https://github.com/mikel/mail/security/advisories/GHSA-mvxr-6m87-mv2q, https://nvd.nist.gov/vuln/detail/CVE-2026-63435, https://github.com/mikel/mail/pull/1664, https://github.com/mikel/mail/commit/f9d59c2e447af42e2c3dec5a56b1bb25c7292859, https://github.com/mikel/mail, https://github.com/mikel/mail/releases/tag/2.9.1