{"id":"CVE-2026-63435","aliases":["GHSA-mvxr-6m87-mv2q"],"url":"https://o3.security/vulnerability/CVE-2026-63435","summary":"Mail: Email address spoofing via malformed RFC 2047 encoded-words","details":"## Summary\n\nMail::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.\n\n## Details\n\nBoth decoders used a single String#match against a pattern such as /\\=\\?(.+)?\\?[Qq]\\?(.*)\\?\\=/m. Two problems:\n\n1. 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.\n2. 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.\n\n## Impact\n\nApplications 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.","published":"2026-09-01T20:09:20.820Z","modified":"2026-09-11T03:48:33.188396073Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"},"epss":{"score":0.00328,"percentile":0.25999,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"mail","fixedVersion":"2.9.1"}],"fix":{"url":"https://github.com/mikel/mail/commit/f9d59c2e447af42e2c3dec5a56b1bb25c7292859","label":"mikel/mail@f9d59c2"},"references":[{"type":"WEB","url":"https://github.com/mikel/mail/releases/tag/2.9.1"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63435.json"},{"type":"ADVISORY","url":"https://github.com/mikel/mail/security/advisories/GHSA-mvxr-6m87-mv2q"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63435"},{"type":"FIX","url":"https://github.com/mikel/mail/commit/f9d59c2e447af42e2c3dec5a56b1bb25c7292859"},{"type":"FIX","url":"https://github.com/mikel/mail/pull/1664"},{"type":"PACKAGE","url":"https://github.com/mikel/mail"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-11T03:48:33.188396073Z"}}