The vulnerability identified as CVE-2026-27942 affects the fast-xml-parser library developed by NaturalIntelligence, specifically versions prior to 5.3.8. This library is widely used in JavaScript environments to parse XML data into JavaScript objects and to build XML from JavaScript objects without relying on C/C++ libraries or callbacks. The vulnerability is a classic buffer overflow (CWE-120) that manifests when the XML builder is used with the 'preserveOrder:true' option. Under these conditions, the application can crash due to a stack overflow caused by unchecked buffer copying during XML building. The flaw arises because the library does not properly check the size of input data before copying it into a buffer, leading to memory corruption and application instability. The vulnerability can be triggered remotely by supplying crafted XML input to the affected XML builder functionality. The issue was resolved in version 5.3.8 of fast-xml-parser. No known exploits have been reported in the wild, and the CVSS v4.0 score is 2.7, reflecting a low severity primarily due to the limited impact scope and lack of authentication or user interaction requirements. The vulnerability primarily results in denial of service through application crashes rather than remote code execution or data compromise.

The primary impact of CVE-2026-27942 is denial of service (DoS) caused by application crashes due to stack overflow when processing XML with the vulnerable fast-xml-parser versions. Organizations relying on this library for XML processing in web applications, APIs, or backend services may experience service interruptions or degraded availability if an attacker supplies malicious XML input with the 'preserveOrder:true' option enabled. Although the vulnerability does not lead to remote code execution or data leakage, the resulting instability can disrupt business operations, especially in environments where XML parsing is critical. The low CVSS score reflects the limited confidentiality and integrity impact, but availability impact may be significant in high-availability or real-time systems. Since no authentication or user interaction is required, attackers can exploit this vulnerability remotely if the affected XML builder functionality is exposed to untrusted input. This risk is heightened in public-facing services or microservices architectures that parse XML from external sources.

To mitigate CVE-2026-27942, organizations should upgrade fast-xml-parser to version 5.3.8 or later, where the vulnerability is fixed. If immediate upgrading is not feasible, the following specific mitigations are recommended: (1) Disable the 'preserveOrder' option in the XML builder by setting it to false, as this prevents the vulnerable code path from being executed. (2) Implement strict input validation and sanitization on all XML data before passing it to the fast-xml-parser builder, ensuring that input size and structure conform to expected parameters to avoid triggering buffer overflows. (3) Employ runtime protections such as application-level sandboxing or memory safety tools to detect and prevent stack overflows. (4) Monitor application logs and crash reports for signs of stack overflow or abnormal termination related to XML processing. (5) Restrict access to XML processing endpoints to trusted sources where possible, reducing exposure to crafted malicious input. (6) Incorporate fuzz testing and static code analysis in the development lifecycle to detect similar vulnerabilities proactively.