Electron: Registry key path injection in app.setAsDefaultProtocolClient on Windows

### Impact On Windows, `app.setAsDefaultProtocolClient(protocol)` did not validate the protocol name before writing to the registry. Apps that pass untrusted input as the protocol name may allow an attacker to write to arbitrary subkeys under `HKCU\Software\Classes\`, potentially hijacking existing protocol handlers. Apps are only affected if they call `app.setAsDefaultProtocolClient()` with a protocol name derived from external or untrusted input. Apps that use a hardcoded protocol name are not affected. ### Workarounds Validate the protocol name matches `/^[a-zA-Z][a-zA-Z0-9+.-]*$/` before passing it to `app.setAsDefaultProtocolClient()`. ### Fixed Versions * `41.0.0` * `40.8.1` * `39.8.1` * `38.8.6` ### For more information If there are any questions or comments about this advisory, please email [security@electronjs.org](mailto:security@electronjs.org)