CVE-2026-25586 is a critical injection vulnerability classified under CWE-74 affecting the nyariv SandboxJS library, a JavaScript sandboxing tool designed to isolate and securely execute untrusted code. Prior to version 0.8.29, SandboxJS improperly neutralizes special elements in output used by downstream components, specifically by allowing sandbox escape through prototype pollution. The root cause is the ability to shadow the hasOwnProperty method on sandbox objects, which disables the enforcement of a prototype whitelist during property access. This flaw permits attackers to access and manipulate the __proto__ property and other blocked prototype properties, effectively polluting the host environment's Object.prototype. Such pollution can lead to persistent cross-sandbox contamination, breaking the isolation guarantees of the sandbox and enabling attackers to execute arbitrary code or escalate privileges within the host environment. The vulnerability is remotely exploitable without requiring authentication or user interaction, making it highly dangerous. The CVSS 3.1 base score is 10.0, reflecting the critical nature of the flaw with complete impact on confidentiality, integrity, and availability. The vulnerability was publicly disclosed on February 6, 2026, and fixed in SandboxJS version 0.8.29. No known exploits have been reported in the wild yet, but the potential for severe damage is significant given the nature of the vulnerability and the widespread use of JavaScript sandboxing in modern web applications and services.

For European organizations, the impact of CVE-2026-25586 can be severe, especially for those relying on SandboxJS for secure execution of untrusted JavaScript code, such as SaaS providers, cloud platforms, and web application developers. Exploitation could lead to full compromise of the sandbox environment, allowing attackers to execute arbitrary code on the host system, access sensitive data, or disrupt services. This can result in data breaches, loss of customer trust, regulatory penalties under GDPR, and operational downtime. Organizations using multi-tenant environments or microservices architectures that depend on sandboxing for isolation are particularly vulnerable to cross-tenant attacks. The critical severity and ease of exploitation without authentication increase the urgency for European entities to address this vulnerability promptly. Additionally, the persistent nature of the Object.prototype pollution can complicate detection and remediation efforts, potentially allowing attackers to maintain long-term access.

European organizations should immediately upgrade all instances of SandboxJS to version 0.8.29 or later to remediate this vulnerability. In addition to patching, organizations should conduct thorough code reviews and security audits of their sandboxing implementations to ensure no other prototype pollution or sandbox escape vectors exist. Implement runtime monitoring and anomaly detection focused on prototype chain modifications and unusual property accesses to detect potential exploitation attempts. Employ defense-in-depth strategies such as strict Content Security Policies (CSP), limiting the privileges of sandboxed code, and isolating critical services to minimize the blast radius of any successful attack. For environments where immediate patching is not feasible, consider disabling or restricting the use of SandboxJS or replacing it with alternative sandboxing solutions with verified security. Regularly update threat intelligence feeds and monitor for emerging exploits targeting this vulnerability. Finally, ensure incident response plans include scenarios involving sandbox escapes and prototype pollution attacks.