Security Digest · Vulnerability record
CVE-2026-25896: fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names
fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback.
CVSS 9.3 Critical EPSS 0.6% · p46
Should you act?
Critical and remotely reachable
No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.6%, percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes
fast-xml-parser(npm) >= 5.0.0, < 5.3.5, fixed in 5.3.5;fast-xml-parser(npm) >= 4.1.3, < 4.5.4, fixed in 4.5.4. Check your lockfile, including transitive dependencies.
What it is
fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. From 4.1.3to before 5.3.5, a 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. This vulnerability is fixed in 5.3.5.
From the GitHub advisory
Entity encoding bypass via regex injection in DOCTYPE entity names
Summary
A 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.
Details
The 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.
Weakness type: CWE-185, CWE-79.
Product: fast-xml-parser (npm) · Assigned by: GitHub_M · Published: 2026-02-20
Affected and fixed versions
| Ecosystem | Package | Vulnerable | First fixed |
|---|---|---|---|
| npm | fast-xml-parser | >= 5.0.0, < 5.3.5 | 5.3.5 |
| npm | fast-xml-parser | >= 4.1.3, < 4.5.4 | 4.5.4 |
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| NaturalIntelligence | fast-xml-parser | >= 5.0.0, < 5.3.5, >= 4.1.3, < 4.5.4 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_M | 3.1 | 9.3 Critical | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N |
| redhat-SADP | 3.1 | 7.1 High | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L |
| GitHub | 3.1 | 9.3 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: none
- Scope: changed (can affect other components)
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc, automatable yes, technical impact partial.
- FIRST EPSS (model of 2026-10-21): 0.57% probability of exploitation activity in the next 30 days; percentile 46.
Timeline
- — CVE record published by GitHub_M
- — GitHub-reviewed advisory GHSA-m7jm-9gc2-mpf2 published (critical)
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
Vulnerability records
- GitHub Advisory GHSA-m7jm-9gc2-mpf2 github.com
- CVE.org record CVE-2026-25896 cve.org
- NVD entry CVE-2026-25896 nvd.nist.gov
Other references from the CVE record and advisory
- github.com github.com
- github.com github.com
- github.com github.com
- github.com github.com
- access.redhat.com access.redhat.com
- RHBZ#2441501 bugzilla.redhat.com
- security.access.redhat.com security.access.redhat.com