The vulnerability in vm2 versions up to 3.11.3 arises from the defaultSandboxPrepareStackTrace function building its output array by assigning to array indices via prototype-walking (lines[lines.length] = value) rather than using a prototype-bypassing define-property method. Because the array is allocated within the sandbox realm, malicious sandbox code can install an accessor on Array.prototype for the relevant index. This accessor is triggered when the sandbox reads error.stack or triggers Error.prepareStackTrace, allowing the sandboxed code to observe and intercept each stack-trace line written by the bridge. The same pattern occurs in the error-handling branch. The vulnerability results in an information side channel and a violation of the sandbox's defense invariant but does not lead to sandbox escape. The vendor rates this issue as low severity. The fix in vm2 3.11.4 replaces the assignment with Reflect.defineProperty to install each entry as an own data property, preventing the interception.

The impact is limited to an information side channel where sandboxed code can observe and intercept stack-trace lines generated by the bridge. There is no known sandbox escape or privilege escalation. The vendor rates the severity as low.

A fix is available in vm2 version 3.11.4, which changes the array property assignment to use Reflect.defineProperty, preventing the interception of stack-trace lines by sandboxed code. Users should upgrade to vm2 3.11.4 or later to remediate this issue.