The vulnerability identified as CVE-2026-25896 affects the fast-xml-parser library developed by NaturalIntelligence, which is widely used to validate, parse, and build XML data in JavaScript environments without relying on native C/C++ libraries or callbacks. The core issue stems from the library's handling of DOCTYPE entity names containing a dot ('.'). Instead of treating the dot as a literal character, the parser interprets it as a regex wildcard during entity replacement. This behavior enables an attacker to shadow or override the built-in XML entities such as <, >, &, ", and ' with arbitrary values. Consequently, the entity encoding intended to prevent injection attacks is bypassed. When the XML is parsed and the output is rendered in a web context, this can lead to cross-site scripting (XSS) vulnerabilities, allowing attackers to execute malicious scripts in the victim's browser. The vulnerability affects fast-xml-parser versions from 4.1.3 up to but not including 4.5.4, and from 5.0.0 up to but not including 5.3.5. The issue does not require any privileges or user interaction to exploit, and the scope is complete as it affects all users of the vulnerable versions. The CVSS v3.1 base score is 9.3 (critical), reflecting the network attack vector, low attack complexity, no privileges required, no user interaction, and a scope change with high impact on integrity and partial impact on confidentiality. No known exploits are currently reported in the wild, but the severity and ease of exploitation make it a significant threat. The vulnerability is addressed in version 5.3.5 of fast-xml-parser.

This vulnerability poses a severe risk to organizations that use the fast-xml-parser library for XML processing in web applications or services. Successful exploitation can lead to cross-site scripting (XSS) attacks, which may allow attackers to execute arbitrary JavaScript in the context of the victim's browser. This can result in session hijacking, credential theft, unauthorized actions on behalf of users, and distribution of malware. Since the vulnerability bypasses entity encoding, it undermines fundamental security assumptions in XML parsing and rendering workflows. The impact extends to any system that processes untrusted XML input and subsequently renders the parsed output in a web interface. Given the widespread use of fast-xml-parser in JavaScript projects, including server-side Node.js applications and client-side tools, the potential attack surface is broad. Organizations handling sensitive data, financial transactions, or critical infrastructure are particularly at risk. The vulnerability does not affect availability but compromises integrity and confidentiality, potentially leading to data breaches and reputational damage.

The primary mitigation is to upgrade the fast-xml-parser library to version 5.3.5 or later, where the issue is fixed. Organizations should audit their codebases and dependencies to identify usage of affected versions and prioritize patching. For environments where immediate upgrade is not feasible, implement strict input validation and sanitization on XML data before parsing, focusing on disallowing or escaping DOCTYPE declarations and entity definitions. Employ Content Security Policy (CSP) headers to restrict the execution of inline scripts and reduce the impact of potential XSS. Additionally, consider using alternative XML parsing libraries that do not exhibit this vulnerability or that have undergone thorough security review. Monitor application logs and web traffic for unusual patterns indicative of attempted exploitation. Finally, educate developers about secure XML handling practices and the risks of improper entity processing.