The vulnerability identified as CVE-2026-26278 affects the fast-xml-parser library developed by NaturalIntelligence, specifically versions from 4.1.3 up to but not including 5.3.6. The root cause is an improper restriction of recursive entity references in Document Type Definitions (DTDs) during XML parsing, classified under CWE-776. This flaw enables an attacker to craft XML inputs containing recursive entity expansions, commonly known as XML Entity Expansion (XEE) attacks. When such malicious XML is processed, the parser recursively expands entities without limits, causing exponential growth in processing time and resource consumption. This leads to a denial-of-service (DoS) condition by freezing or significantly slowing down the application handling the XML. The vulnerability is exploitable remotely without requiring any privileges or user interaction, increasing its risk profile. The fast-xml-parser library is widely used in JavaScript environments for XML validation, parsing, and building XML from JS objects without relying on native C/C++ libraries. The issue was addressed in version 5.3.6 by restricting entity expansion. As an interim workaround, disabling DOCTYPE parsing via the processEntities: false option can prevent exploitation. No known active exploits have been reported in the wild as of the publication date. The CVSS v3.1 base score is 7.5, reflecting high severity due to the ease of exploitation and significant impact on availability.

The primary impact of CVE-2026-26278 is a denial-of-service condition caused by resource exhaustion during XML parsing. Organizations using vulnerable versions of fast-xml-parser in web applications, APIs, or backend services that process XML inputs are at risk of application freezes or crashes, leading to service unavailability. This can disrupt business operations, degrade user experience, and potentially cause cascading failures in dependent systems. Since exploitation requires no authentication and can be triggered remotely, attackers can easily launch DoS attacks at scale. While confidentiality and integrity are not directly affected, the availability impact alone can be critical for services relying heavily on XML processing, such as financial platforms, enterprise integrations, and cloud services. The vulnerability could also be leveraged as part of a multi-vector attack to distract or exhaust resources while other attacks are conducted. Organizations with high XML traffic or exposed XML endpoints are particularly vulnerable.

To mitigate CVE-2026-26278, organizations should upgrade fast-xml-parser to version 5.3.6 or later, where the vulnerability is fixed. If immediate upgrading is not feasible, configure the parser to disable DOCTYPE parsing by setting the processEntities option to false, effectively preventing entity expansion attacks. Additionally, implement input validation and filtering to reject XML inputs containing DOCTYPE declarations or suspicious entity definitions. Employ resource limits and timeouts on XML parsing operations to prevent excessive CPU consumption. Monitoring and alerting on unusually high CPU or memory usage in services handling XML can help detect attempted exploitation. For critical systems, consider isolating XML parsing in sandboxed environments to contain potential DoS effects. Regularly review dependencies and apply security patches promptly to reduce exposure to such vulnerabilities.