Cyber Resilience

CVE-2026-25896

XSS in Naturalintelligence Fast-Xml-Parser 4.1.3 – 5.3.5

Public PoCXSS
Published
20 February 2026
Modified
12 August 2026
Patch / advisory
View advisory
CVSS Score v3.1 9.3
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N
EPSS Score 0.0046 38th percentile
Risk Priority 66 floored blend · peak EPSS

Summary

CVE-2026-25896 is a critical-severity Incorrect Regular Expression (CWE-185) vulnerability in Naturalintelligence Fast-Xml-Parser. Its CVSS base score is 9.3 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Browser Session Hijacking (T1185); ranked at the 38th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2026-25896 affects the fast-xml-parser JavaScript library, versions 4.1.3 through 5.3.4, which is used to validate XML, parse XML into JavaScript objects, or build XML from JavaScript objects without C/C++ dependencies or callbacks. The vulnerability arises because a dot (.) character in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, enabling attackers to shadow built-in XML entities such as <, >, &, ", and ' with arbitrary values. This flaw, classified under CWE-185, bypasses standard entity encoding and has a CVSS v3.1 base score of 9.3 (AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N).

An attacker can exploit this vulnerability by supplying malicious XML input containing a crafted DOCTYPE entity with a dot in its name to any application using the affected fast-xml-parser versions for untrusted XML processing. No privileges or user interaction are required, and exploitation occurs over the network with low complexity. Successful exploitation allows the attacker to inject arbitrary content by overriding built-in entities, leading to cross-site scripting (XSS) when the parsed XML output is rendered in a web context, with high integrity impact due to the scope change.

The vulnerability is fixed in fast-xml-parser version 5.3.5, as detailed in the project's GitHub security advisory (GHSA-m7jm-9gc2-mpf2) and release notes. Mitigation involves upgrading to 5.3.5 or later, with relevant patches in commits 943ef0eb1b2d3284e72dd74f44a042ee9f07026e and ddcd0acf26ddd682cb0dc15a2bd6aa3b96bb1e69. Security practitioners should review dependencies in Node.js or browser-based applications handling XML for vulnerable versions.

OWASP Top 10 for Web (2025)

A05:2025 Injection

EU & UK References

Vulnerability Data

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…

more

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.

CWE(s)
CWE-185CWE-79

Related Threats

MITRE ATT&CK Enterprise Techniques

T1185 Browser Session Hijacking Collection
Adversaries may take advantage of security vulnerabilities and inherent functionality in browser software to change content, modify user-behaviors, and intercept information as part of various browser session hijacking techniques.
attack.mitre.org →
T1539 Steal Web Session Cookie Credential Access
An adversary may steal web application or service session cookies and use them to gain access to web applications or Internet services as an authenticated user without needing credentials.
attack.mitre.org →
T1659 Content Injection Initial Access
Adversaries may gain access and continuously communicate with victims by injecting malicious content into systems through online network traffic.
attack.mitre.org →
T1059 Command and Scripting Interpreter Execution
Adversaries may abuse command and script interpreters to execute commands, scripts, or binaries.
attack.mitre.org →
T1189 Drive-by Compromise Initial Access
Adversaries may gain access to a system through a user visiting a website over the normal course of browsing.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-25837Shared CWE-79
CVE-2023-22438Shared CWE-79
CVE-2023-27614Shared CWE-79
CVE-2023-47164Shared CWE-79
CVE-2023-49145Shared CWE-79
CVE-2023-47853Shared CWE-79
CVE-2023-28474Shared CWE-79
CVE-2023-37636Shared CWE-79
CVE-2023-1006Shared CWE-79
CVE-2023-5005Shared CWE-79

Affected Assets

naturalintelligence
fast-xml-parser
4.1.3 — 5.3.5

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.2
  • V1.3.2

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation can discover regex that fails to match or compare data as intended.

SI-10 Information Input Validation good match

Input validation directly enforces neutralization of untrusted data before it reaches web output generation.

SI-15 Information Output Filtering good match

Output filtering can catch or sanitize unneutralized script content before it is served to users.

SA-15 Development Process, Standards, and Tools partial match

Requiring documented development standards and tools reduces the chance that incorrect regular expressions are written into validation or filtering logic.

SA-3 System Development Life Cycle partial match

An SDLC that incorporates security can embed reviews or tooling that catch malformed regular expressions before deployment.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require correct regex design, testing, and review to avoid improper matching.

PR.PS-02 partial match
prevents

Patching and EOL replacement can remediate known XSS instances in libraries or frameworks (partial) but do nothing to enforce input neutralization in application code (none).

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development can detect regex-related flaws before release.

A.5.6 Contact with special interest groups partial match
prevents

Knowledge exchange on emerging attack techniques and patches reduces the likelihood that cross-site scripting flaws remain unaddressed in deployed applications.

A.5.7 Threat intelligence partial match
prevents

Operational indicators of compromise for web-application attacks can be incorporated into WAF or input-filtering rules, lowering the likelihood that unsanitized data reaches the browser.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle includes validation of input-handling constructs such as regex.

A.8.26 Application security requirements partial match
prevents

Application security requirements can mandate correct regex design and testing.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly address proper construction and review of regular expressions.

References