TeleJSON: DOM XSS via unsanitised constructor name in `new Function()`

Summary

telejson versions prior to 6.0.0 (released 2022) are vulnerable to DOM-based Cross-Site Scripting (XSS) through unsafe deserialisation. Attacker-controlled input from the _constructor-name_ property in parsed JSON is passed directly to new Function() without sanitisation, allowing arbitrary JavaScript execution.

Affected versions

Details

telejson's parse() function uses a custom reviver to reconstruct JavaScript objects from serialised JSON. When processing objects with a _constructor-name_ property, the reviver passes the constructor name directly to new Function() to recreate the object's prototype.

In versions prior to 6.0.0, this constructor name is not sanitised. An attacker who can deliver a crafted JSON payload to telejson.parse() (for example, via postMessage in applications that use telejson for cross-frame communication) can inject arbitrary JavaScript into the new Function() call.

Vulnerable code (src/index.ts, lines 293-299 at v5.3.3):

if (isObject<ValueContainer>(value) && value['_constructor-name_']) {
  const name = value['_constructor-name_'];
  if (name !== 'Object') {
    const Fn = new Function(`return function ${name}(){}`)();
    Object.setPrototypeOf(value, new Fn());
  }

Fixed code (src/index.ts, lines 340-346 at v6.0.0):

if (isObject<ValueContainer>(value) && value['_constructor-name_'] && options.allowFunction) {
  const name = value['_constructor-name_'];
  if (name !== 'Object') {
    const Fn = new Function(`return function ${name.replace(/[\W_]+/g, '')}(){}`)();
    Object.setPrototypeOf(value, new Fn());
  }

The fix introduces two mitigations: a character allowlist via regex that strips non-word characters before they reach new Function(), and gating the entire code path behind the allowFunction option.

Impact

An attacker can execute arbitrary JavaScript in the context of the application using the vulnerable telejson version. Depending on the application, this could enable session hijacking, credential theft, or arbitrary DOM manipulation.

Remediation

Upgrade to telejson >= 6.0.0.