{"id":"CVE-2026-32762","aliases":["GHSA-qfgr-crr9-7r49"],"url":"https://o3.security/vulnerability/CVE-2026-32762","summary":"Rack: Forwarded Header semicolon injection enables Host and Scheme spoofing","details":"## Summary\n\n`Rack::Utils.forwarded_values` parses the RFC 7239 `Forwarded` header by splitting on semicolons before handling quoted-string values. Because quoted values may legally contain semicolons, a header such as:\n\n```http\nForwarded: for=\"127.0.0.1;host=evil.com;proto=https\"\n```\n\ncan be interpreted by Rack as multiple `Forwarded` directives rather than as a single quoted `for` value.\n\nIn deployments where an upstream proxy, WAF, or intermediary validates or preserves quoted `Forwarded` values differently, this discrepancy can allow an attacker to smuggle `host`, `proto`, `for`, or `by` parameters through a single header value.\n\n## Details\n\n`Rack::Utils.forwarded_values` processes the header using logic equivalent to:\n\n```ruby\nforwarded_header.split(';').each_with_object({}) do |field, values|\n  field.split(',').each do |pair|\n    pair = pair.split('=').map(&:strip).join('=')\n    return nil unless pair =~ /\\A(by|for|host|proto)=\"?([^\"]+)\"?\\Z/i\n    (values[$1.downcase.to_sym] ||= []) << $2\n  end\nend\n```\n\nThe method splits on `;` before it parses individual `name=value` pairs. This is inconsistent with RFC 7239, which permits quoted-string values, and quoted strings may contain semicolons as literal content.\n\nAs a result, a header value such as:\n\n```http\nForwarded: for=\"127.0.0.1;host=evil.com;proto=https\"\n```\n\nis not treated as a single `for` value. Instead, Rack may interpret it as if the client had supplied separate `for`, `host`, and `proto` directives.\n\nThis creates an interpretation conflict when another component in front of Rack treats the quoted value as valid literal content, while Rack reparses it as multiple forwarding parameters.\n\n## Impact\n\nApplications that rely on `Forwarded` to derive request metadata may observe attacker-controlled values for `host`, `proto`, `for`, or related URL components.\n\nIn affected deployments, this can lead to host or scheme spoofing in derived values such as `req.host`, `req.scheme`, `req.base_url`, or `req.url`. Applications that use those values for password reset links, redirects, absolute URL generation, logging, IP-based decisions, or backend requests may be vulnerable to downstream security impact.\n\nThe practical security impact depends on deployment architecture. If clients can already supply arbitrary trusted `Forwarded` parameters directly, this bug may not add meaningful attacker capability. The issue is most relevant where an upstream component and Rack interpret the same `Forwarded` header differently.\n\n## Mitigation\n\n* Update to a patched version of Rack that parses `Forwarded` quoted-string values before splitting on parameter delimiters.\n* Avoid trusting client-supplied `Forwarded` headers unless they are normalized or regenerated by a trusted reverse proxy.\n* Prefer stripping inbound `Forwarded` headers at the edge and reconstructing them from trusted proxy metadata.\n* Avoid using `req.host`, `req.scheme`, `req.base_url`, or `req.url` for security-sensitive operations unless the forwarding chain is explicitly trusted and validated.","published":"2026-04-02T17:06:50.819Z","modified":"2026-08-12T03:51:39.038228057Z","cvss":{"score":4.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"},"epss":{"score":0.00179,"percentile":0.07669,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"rack","fixedVersion":"3.1.21"},{"ecosystem":"RubyGems","name":"rack","fixedVersion":"3.2.6"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/32xxx/CVE-2026-32762.json"},{"type":"ADVISORY","url":"https://github.com/rack/rack/security/advisories/GHSA-qfgr-crr9-7r49"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-32762"},{"type":"PACKAGE","url":"https://github.com/rack/rack"},{"type":"WEB","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-32762.yml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:39.038228057Z"}}