{"vulnerability": "CVE-2026-63435", "sightings": [{"uuid": "f1d04dc9-7da2-4601-bf45-7a003467f47e", "vulnerability_lookup_origin": "1a89b78e-f703-45f3-bb86-59eb712668bd", "author": "9f56dd64-161d-43a6-b9c3-555944290a09", "vulnerability": "CVE-2026-63435", "type": "seen", "source": "https://gist.github.com/alon710/29f68a70244d7a5251761f7cfc9eeaab", "content": "# CVE-2026-63435: CVE-2026-63435: Parser Interpretation Conflict in Ruby Mail Gem RFC 2047 Decoders\n\n> **CVSS Score:** 5.3\n> **Published:** 2026-09-02\n> **Full Report:** https://cvereports.com/reports/CVE-2026-63435\n\n## Summary\nAn 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.\n\n## TL;DR\nThe 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.\n\n## Exploit Status: POC\n\n## Technical Details\n\n- **CWE ID**: CWE-436\n- **Attack Vector**: Network\n- **CVSS Score**: 5.3\n- **EPSS Score**: 0.00328\n- **Impact**: Partial Integrity (Bypass of Email Validation Frameworks)\n- **Exploit Status**: Proof-of-Concept\n- **KEV Status**: Not Listed\n\n## Affected Systems\n\n- Ruby on Rails applications utilizing 'mail' or 'actionmailer' dependencies\n- Standalone Ruby applications and scripts utilizing the 'mail' gem for parsing raw incoming emails\n- Email processing pipelines and security filters running vulnerable gem versions\n- **mail (Ruby Gem)**: < 2.9.1 (Fixed in: `2.9.1`)\n\n## Mitigation\n\n- Upgrade the Ruby 'mail' gem dependency to version 2.9.1 or higher.\n- Configure intermediate Mail Transfer Agents (MTAs) to block or sanitize headers containing malformed RFC 2047 tokens.\n- Implement application-level validations to reject header strings that retain raw encoded-word delimiters after parsing.\n\n**Remediation Steps:**\n1. Identify all Ruby projects using the 'mail' gem by checking Gemfile.lock files.\n2. Modify the Gemfile to require gem 'mail', '>= 2.9.1'.\n3. Execute 'bundle update mail' within the application directories.\n4. Run automated test suites to ensure email header parsing operates normally with the patched logic.\n5. Deploy the updated application build to staging and production environments.\n\n## References\n\n- [GitHub Security Advisory: GHSA-mvxr-6m87-mv2q](https://github.com/mikel/mail/security/advisories/GHSA-mvxr-6m87-mv2q)\n- [CVE.org Record - CVE-2026-63435](https://www.cve.org/CVERecord?id=CVE-2026-63435)\n- [NVD Vulnerability Detail - CVE-2026-63435](https://nvd.nist.gov/vuln/detail/CVE-2026-63435)\n\n\n---\n*Generated by [CVEReports](https://cvereports.com/reports/CVE-2026-63435) - Automated Vulnerability Intelligence*", "creation_timestamp": "2026-09-03T13:31:46.838959Z"}]}