{"id":"CVE-2026-126206","aliases":[],"url":"https://o3.security/vulnerability/CVE-2026-126206","summary":"Loofah allowed_uri? treats data:text/html as safe text/plain","details":"## Summary\n\n`Loofah::HTML5::Scrub.allowed_uri?` returns `true` for a `data:` URI whose mediatype is written with the HTML5 named character reference `/` in place of the solidus. Loofah reads that mediatype as malformed and falls back to the allowed `text/plain`, while a browser decodes the reference and reads the same URI as `text/html`.\n\n## Details\n\n`allowed_uri?` decodes HTML entities with `CGI.unescapeHTML`, which handles numeric character references but not HTML5 named character references. `/` therefore survives normalization, so the mediatype carries no `/`, fails the MIME grammar, and falls back to the allowed `text/plain` under the WHATWG rule for a malformed mediatype.\n\n`:`, `/`, `	` and `
` are the only named character references in the HTML5 table that decode to characters that change how a URI parses (U+003A, U+002F, U+0009 and U+000A; there is no named reference for U+000D), so this closes the set.\n\nLoofah's default `sanitize()` path is **not** affected, because Nokogiri decodes the entity during parsing before Loofah evaluates the URI. This issue affects only callers of the public `allowed_uri?` string-level helper that pass it HTML-encoded strings.\n\n## Impact\n\nApplications that validate a user-controlled URL with `Loofah::HTML5::Scrub.allowed_uri?` and then render the approved value into an `href` or another browser-interpreted URI attribute may be vulnerable to cross-site scripting. `Rails::HTML::Sanitizer.allowed_uri?` re-exports the same helper.\n\n## Mitigation\n\nUpgrade to Loofah >= 2.25.3.\n\n## Credit\n\nResponsibly reported by GitHub user @ngpestelos.","published":"2026-11-12T20:56:30Z","modified":"2026-11-12T21:00:03.782376099Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"RubyGems","name":"loofah","fixedVersion":"2.25.3"}],"fix":{"url":"https://github.com/flavorjones/loofah/pull/324","label":"flavorjones/loofah#324"},"references":[{"type":"WEB","url":"https://github.com/flavorjones/loofah/security/advisories/GHSA-jcff-cj73-4w4f"},{"type":"WEB","url":"https://github.com/flavorjones/loofah/pull/324"},{"type":"WEB","url":"https://github.com/flavorjones/loofah/commit/ad92002966053309f83b550861e2ef04bc714744"},{"type":"PACKAGE","url":"https://github.com/flavorjones/loofah"},{"type":"WEB","url":"https://github.com/flavorjones/loofah/releases/tag/v2.25.3"},{"type":"WEB","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/loofah/GHSA-jcff-cj73-4w4f.yml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-11-12T21:00:03.782376099Z"}}