GHSA-2gg9-6p7w-6cpj

Summary

SandboxJS blocks direct assignment to global objects (for example Math.random = ...), but this protection can be bypassed through an exposed callable constructor path: this.constructor.call(target, attackerObject). Because this.constructor resolves to the internal SandboxGlobal function and Function.prototype.call is allowed, attacker code can write arbitrary properties into host global objects and persist those mutations across sandbox instances in the same process.

Details

The intended safety model relies on write-time checks in assignment operations. In assignCheck, writes are denied when the destination is marked global (obj.isGlobal), which correctly blocks straightforward payloads like Math.random = () => 1.

Reference: src/executor.ts#L215-L218

if (obj.isGlobal) {
  throw new SandboxAccessError(
    `Cannot ${op} property '${obj.prop.toString()}' of a global object`,
  );
}

The bypass works because the dangerous write is not performed by an assignment opcode. Instead, attacker code reaches a host callable that performs writes internally. The constructor used for sandbox global objects is SandboxGlobal, implemented as a function that copies all keys from a provided object into this.

Reference: src/utils.ts#L84-L88

export const SandboxGlobal = function SandboxGlobal(this: ISandboxGlobal, globals: IGlobals) {
  for (const i in globals) {
    this[i] = globals[i];
  }
} as any as SandboxGlobalConstructor;

At runtime, global scope this is a SandboxGlobal instance (functionThis), so this.constructor resolves to SandboxGlobal. That constructor is reachable from sandbox code, and calls through Function.prototype.call are allowed by the generic call opcode path.

References: -...