CVE-2026-63435 Mail: Email address spoofing via malformed RFC 2047 encoded-words
Attackers can spoof email headers via malformed RFC 2047 encoding, bypassing security checks and enabling phishing in applications using the Ruby Mail library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63435?
CVE-2026-63435 is a medium-severity vulnerability in mikel mail, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-436
Which versions are affected by CVE-2026-63435?
2 open source packages ship an affected version, including mail.
| Package | Affected | Fixed in |
|---|---|---|
mail RubyGems | All versions | 2.9.1 |
mail RubyGems | All versions | 2.9.1 |
How do I fix CVE-2026-63435?
Upgrade mail to 2.9.1 or later.
Is CVE-2026-63435 being exploited?
There is no known exploitation of CVE-2026-63435 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-JD97527, CLEANSTART-2026-UV37642, DEBIAN-CVE-2026-63435, GHSA-mvxr-6m87-mv2q, MINI-3qgm-rhmq-5jh9, MINI-8497-x9jj-x597 and 13 more.