CVE-2026-63435: CVE-2026-63435: Parser Interpretation Conflict in Ruby Mail Gem RFC 2047 Decoders

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CVE-2026-63435: Parser Interpretation Conflict in Ruby Mail Gem RFC 2047 Decoders

Vulnerability ID: CVE-2026-63435
CVSS Score: 5.3
Published: 2026-09-02

An interpretation conflict (CWE-436) exists in the Ruby 'mail' library's RFC 2047 decoding implementation. Vulnerable versions utilize regular expressions with overly greedy qualifiers and a singular matching strategy. When parsing malformed headers, these design flaws trigger unexpected exception-handling behaviors, outputting raw, unparsed strings. Consequently, intermediate security gateways and downstream Ruby processors interpret email addresses differently, enabling authentication bypasses, phishing, and header spoofing.

TL;DR

The Ruby 'mail' library prior to version 2.9.1 contains a regex and parsing flaw in its RFC 2047 decoding logic, allowing malformed headers to bypass security validations and cause discrepancy exploits between mail agents.

⚠️ Exploit Status: POC

Technical Details

CWE ID: CWE-436
Attack Vector: Network
CVSS Score: 5.3
EPSS Score: 0.00328
Impact: Partial Integrity (Bypass of Email Validation Frameworks)
Exploit Status: Proof-of-Concept
KEV Status: Not Listed

Affected Systems

Ruby on Rails applications utilizing 'mail' or 'actionmailer' dependencies
Standalone Ruby applications and scripts utilizing the 'mail' gem for parsing raw incoming emails
Email processing pipelines and security filters running vulnerable gem versions
mail (Ruby Gem): < 2.9.1 (Fixed in: 2.9.1)

Code Analysis

Commit: f9d59c2

Hardened RFC 2047 utility decoding functions (b_value_decode and q_value_decode) to use global replacement (gsub) and non-greedy regex matching to prevent parser differential-based interpretation conflicts.

Mitigation Strategies

Upgrade the Ruby 'mail' gem dependency to version 2.9.1 or higher.
Configure intermediate Mail Transfer Agents (MTAs) to block or sanitize headers containing malformed RFC 2047 tokens.
Implement application-level validations to reject header strings that retain raw encoded-word delimiters after parsing.

Remediation Steps:

Identify all Ruby projects using the 'mail' gem by checking Gemfile.lock files.
Modify the Gemfile to require gem 'mail', '>= 2.9.1'.
Execute 'bundle update mail' within the application directories.
Run automated test suites to ensure email header parsing operates normally with the patched logic.
Deploy the updated application build to staging and production environments.

References

Read the full report for CVE-2026-63435 on our website for more details including interactive diagrams and full exploit analysis.

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