A significant security flaw has been identified in the JavaScript library SandboxJS, posing a threat to host systems by allowing potential remote code execution. This vulnerability, known as CVE-2026-25881, has a critical CVSS score of 8.3/10 and affects all iterations up to version 0.8.30, with a patch available in version 0.8.31.
Understanding the SandboxJS Vulnerability
The discovered flaw originates from a loophole in the protection mechanism of SandboxJS, specifically its use of an “isGlobal” flag intended to prevent sandboxed scripts from altering host system objects. However, this security measure can be bypassed through a technique known as “prototype pollution.” By manipulating global prototype references like Map.prototype within an array, the protective flag is inadvertently removed.
Such a breach allows attackers to alter core JavaScript objects persistently. For instance, a malicious command can be injected into the Map prototype, which then affects all instances of Map objects created by the host application. This manipulation can lead to severe outcomes if the host application subsequently utilizes these compromised properties during sensitive tasks.
Exploitation and Demonstrations
Security researcher k14uz has released a proof-of-concept exploit demonstrating how this vulnerability can be leveraged in three different attack scenarios. The simplest example involves adding a “polluted” property to the Map prototype, which then becomes a part of every new Map object.
More advanced demonstrations showcase the ability to override built-in functions and execute commands like “id” to extract user information. The attack sequence is straightforward: create an array with a prototype reference, extract it to negate the protection flag, modify the prototype, and finally, wait for the host code to use these altered properties.
Implications and Mitigation Strategies
Any application employing SandboxJS to execute untrusted JavaScript code is at risk. Attackers can breach sandbox isolation, alter host execution paths, and potentially seize full control of the system based on how the host manages these compromised properties.
To mitigate these risks, developers are urged to upgrade to SandboxJS version 0.8.31 immediately. This update ensures that protection flags are maintained across array operations and prohibits modifications to built-in prototypes. Additionally, security teams should consider freezing built-in prototypes before running untrusted scripts and auditing applications for vulnerable operations involving user-controlled properties.
This vulnerability highlights the complexities involved in creating secure JavaScript sandboxes, emphasizing the need for robust, multi-layered security measures when dealing with untrusted code. Stay informed with daily cybersecurity updates by following us on Google News, LinkedIn, and X. Contact us to share your cybersecurity stories.
