CVE-2026-26278
Naturalintelligence Fast-Xml-Parser 4.1.3 – 5.3.6
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HSummary
CVE-2026-26278 is a high-severity XML Entity Expansion (CWE-776) vulnerability in Naturalintelligence Fast-Xml-Parser. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Endpoint Denial of Service (T1499); ranked in the top 45% of CVEs by exploit likelihood; 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 CM-6 (Configuration Settings) 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-2026-26278 is a denial-of-service vulnerability in the fast-xml-parser JavaScript library, which enables XML validation, parsing to JavaScript objects, or building XML from objects without C/C++ dependencies or callbacks. Affecting versions 4.1.3 through 5.3.5, the flaw (CWE-776) allows unlimited entity expansion during XML parsing. A small malicious XML input can trigger excessive processing, causing the parser to spend seconds or minutes on a single request and effectively freeze the application. The vulnerability carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).
Any unauthenticated attacker with network access to an application using the vulnerable fast-xml-parser versions can exploit this remotely. By crafting a concise XML payload with expansive or recursive entity definitions, the attacker induces the parser to perform unbounded expansions, leading to high CPU consumption and temporary denial of service without requiring privileges or user interaction.
The vulnerability is addressed in fast-xml-parser version 5.3.6. As a workaround, disable DOCTYPE entity processing via the `processEntities: false` option. Additional details are available in the GitHub security advisory (GHSA-jmr7-xgp7-cmfj), the fixing commit (910dae5be2de2955e968558fadf6e8f74f117a77), and the v5.3.6 release notes.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-7970
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. In versions 4.1.3 through 5.3.5, the XML parser can be forced to do an unlimited amount…
more
of entity expansion. With a very small XML input, it’s possible to make the parser spend seconds or even minutes processing a single request, effectively freezing the application. Version 5.3.6 fixes the issue. As a workaround, avoid using DOCTYPE parsing by `processEntities: false` option.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Secure configuration settings can disable DTD processing or cap entity expansion in XML parsers.
Input validation can enforce limits on XML entity expansion and recursion depth before parsing occurs.
Least-functionality configuration can prohibit unsafe DTD features or external entity resolution.
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.
Secure SDLC practices directly prevent introduction of improper recursive entity handling in XML code.
Hardened configuration baselines can disable or limit DTD entity expansion in XML parsers.
Identifying and recording vulnerabilities can discover XML entity expansion flaws before exploitation.
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.
Security testing can detect and block XML entity-expansion vulnerabilities before deployment.
Secure SDLC mandates input validation and parser hardening that can prevent unbounded entity expansion.
Application security requirements can explicitly call for limits on XML entity recursion and DTD processing.
Secure architecture principles discourage unsafe parser configurations that enable entity expansion attacks.
Secure coding standards directly require disabling or restricting recursive entity references in XML parsers.