The vulnerability identified as CVE-2026-27942 affects the fast-xml-parser library developed by NaturalIntelligence, specifically versions before 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 root cause is a classic buffer overflow (CWE-120) triggered when the XML builder is used with the 'preserveOrder:true' option. Under these conditions, the application attempts to copy data into a buffer without properly checking the input size, leading to a stack overflow and subsequent crash. The vulnerability does not require any authentication or user interaction, making it remotely exploitable if the XML builder is exposed to untrusted input. The CVSS 4.0 vector indicates network attack vector, low complexity, no privileges or user interaction required, and low impact on confidentiality and integrity but some impact on availability due to application crashes. The issue was resolved in version 5.3.8 by implementing proper input size checks. Until upgrading, users can mitigate risk by disabling 'preserveOrder' or validating input data before passing it to the builder. No public exploits have been reported, suggesting limited active exploitation currently.

The primary impact of this vulnerability is denial of service (DoS) caused by application crashes due to stack overflow. For organizations relying on fast-xml-parser in server-side or client-side applications, this can lead to service interruptions, degraded user experience, or potential downtime. Since the vulnerability does not appear to allow arbitrary code execution or data leakage, the confidentiality and integrity impacts are minimal. However, availability impact can be significant if the XML builder with 'preserveOrder:true' is used in processing untrusted or malformed XML inputs. Attackers could craft malicious XML payloads to trigger crashes, potentially disrupting critical services or automated workflows that depend on XML parsing. The low CVSS score reflects the limited scope and impact, but organizations with high availability requirements should treat this vulnerability seriously. The absence of known exploits reduces immediate risk but does not eliminate the potential for future exploitation.

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, disable the 'preserveOrder' option in the XML builder to avoid triggering the overflow condition. Additionally, implement strict input validation and sanitization on all XML data before processing to ensure no oversized or malformed inputs reach the builder. Employ runtime protections such as stack canaries and address space layout randomization (ASLR) to reduce the risk of exploitation. Monitor application logs for crashes or unusual behavior related to XML parsing. In environments where XML input comes from untrusted sources, consider isolating the parsing process or using sandboxing techniques to limit potential damage. Regularly review dependencies and apply security patches promptly to minimize exposure.