CVE-2026-63435

Received Received - Intake

Mail::Utilities Decoding Flaw Enables Header Spoofing

Publication date: 2026-09-01

Last updated on: 2026-09-01

Assigner: GitHub, Inc.

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.

CVSS Scores

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Meta Information

Published

2026-09-01

Last Modified

2026-09-01

Generated

2026-09-02

AI Q&A

2026-09-02

EPSS Evaluated

N/A

NVD

CVE-2026-63435

Affected Vendors & Products

Vendor	Product	Version / Range
mail	mail	2.9.1

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-436	Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.

Attack-Flow Graph

Executive Summary

This vulnerability affects the Ruby Mail library before version 2.9.1. It involves improper handling of RFC 2047 encoded-words in email headers. The functions Mail::Utilities.q_value_decode and Mail::Utilities.b_value_decode use a greedy charset capture that incorrectly decodes only the first encoded-word and mishandles surrounding text. This allows crafted malformed encoded-words to alter decoded From, To, or Reply-To header values without changing the raw text visible to humans or downstream parsers.

Detection Guidance

This vulnerability can be detected by inspecting email headers for malformed RFC 2047 encoded-words in From, To, or Reply-To fields. Check Ruby Mail library versions below 2.9.1. Use commands like gem list mail to verify installed versions.

Impact Analysis

This vulnerability could enable sender or recipient spoofing in emails, making phishing attacks more convincing. It might also allow bypassing authorization checks by manipulating email headers. Users relying on the Mail library for email processing in Ruby applications could be affected if they use versions prior to 2.9.1.

Compliance Impact

This vulnerability could enable sender or recipient spoofing in email headers, which may lead to phishing attacks or authorization-check bypasses. Such spoofing could undermine data integrity and authenticity requirements in GDPR and HIPAA, potentially violating compliance by allowing unauthorized access or misleading data handling.

Mitigation Strategies

Upgrade the Ruby Mail library to version 2.9.1 or later. If using Bundler, run bundle update mail. Review email headers for signs of spoofing attempts. Monitor for phishing or unauthorized access attempts.

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