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 4.5.4, and from 5.0.0 up to but not including 5.3.6. The root cause is an improper restriction of recursive entity references in XML Document Type Definitions (DTDs), classified under CWE-776. This flaw allows an attacker to craft XML input containing recursive entity expansions, which the parser processes without limitation. As a result, even a small XML payload can cause the parser to perform an exponential number of entity expansions, consuming excessive CPU resources and causing the application to freeze or become unresponsive. This type of attack is commonly known as an XML Entity Expansion (XEE) or 'billion laughs' attack. The vulnerability affects the parser's ability to safely handle XML inputs that include DOCTYPE declarations with entities. The fast-xml-parser library is widely used in JavaScript environments to convert XML to JavaScript objects and vice versa without relying on native C/C++ libraries. The vulnerability does not require any authentication or user interaction and can be triggered remotely by submitting malicious XML data to applications using the affected parser versions. The issue was publicly disclosed on February 19, 2026, with a CVSS v3.1 base score of 7.5 (high severity), reflecting its potential to cause denial of service (availability impact) without affecting confidentiality or integrity. The vendor fixed the issue in version 5.3.6. As a mitigation, users can disable entity processing by setting the parser option processEntities to false, which prevents DOCTYPE parsing and entity expansion, thereby avoiding the vulnerability.

This vulnerability primarily impacts the availability of applications using the affected versions of fast-xml-parser. An attacker can exploit the flaw to cause a denial of service by sending a crafted XML payload that triggers excessive entity expansion, leading to high CPU consumption and application freezing. This can disrupt services, degrade user experience, and potentially cause cascading failures in systems relying on the parser for XML processing. Since fast-xml-parser is commonly used in web applications, APIs, and backend services that handle XML data, the attack surface is broad. Organizations processing untrusted XML inputs are at particular risk. Although the vulnerability does not compromise data confidentiality or integrity, the denial of service can result in operational downtime, financial loss, and reputational damage. The ease of exploitation (no authentication or user interaction required) and remote attack vector increase the threat level. The absence of known exploits in the wild at the time of disclosure suggests limited active exploitation, but the high severity score warrants prompt remediation.

To mitigate this vulnerability, organizations should immediately upgrade fast-xml-parser to version 5.3.6 or later, where the issue is fixed. If upgrading is not immediately feasible, a practical workaround is to disable DOCTYPE parsing by setting the parser option processEntities to false, which prevents entity expansion and eliminates the attack vector. Additionally, implement input validation and filtering to reject XML inputs containing DOCTYPE declarations or suspicious entity references. Employ rate limiting and resource usage monitoring on services processing XML to detect and mitigate potential denial of service attempts. Consider using XML parsers with built-in protections against entity expansion attacks or sandbox XML processing in isolated environments to minimize impact. Regularly review dependencies for updates and security advisories to stay ahead of similar vulnerabilities.