Sparkle is a software update framework for macOS. In versions up to and including 2.9.1, `Autoupdate/AppInstaller.m`'s `shouldAcceptNewConnection:` only enforces `SUCodeSigningVerifier validateConnection:` before stage 1 completes. After `_performedStage1Installation = YES`, new connections to the registered Mach service `<bundleId>-spki` are accepted from any local process without team-ID or code-signing checks. As of time of publication, no known patched versions are available.

Affected: SwiftURL: github.com/sparkle-project/Sparkle

## Summary

AppInstaller post-stage-1 XPC listener accepts unvalidated connections, allowing spoofed appcast item data injection.

## Details

`Autoupdate/AppInstaller.m`'s `shouldAcceptNewConnection:` only enforces `SUCodeSigningVerifier validateConnection:` before stage 1 completes. After `_performedStage1Installation = YES`, new connections to the registered Mach service `<bundleId>-spki` are accepted from any local process without team-ID or code-signing checks.

The following chain of events enables an attacker to inject a spoofed `SPUSentUpdateAppcastItemData` payload:

1. Installer finishes unarchiving the update successfully (`_willCompleteInstallation` is set). 2. The app responsible for updating the bundle crashes or is forcefully quit before it has a chance to send `SPUSentUpdateAppcastItemData` to the installer. There is no user interaction between the prior step and this one, so the timing window is tight. 3. After stage 1 of the installer is performed (`_performedStage1Installation = YES`), but before final installation completes (since all services are cleaned up by then), an attacker process connects to the `<bundleId>-spki` Mach service - no code-signing validation is enforced - and sends a spoofed `SPUSentUpdateAppcastItemData` message containing an attacker-crafted `SUAppcastItem`. 4. A Sparkle-aware app that checks for updates on the bundle being updated launches before installation completes. The progress agent re-broadcasts the spoofed `SUAppcastItem` o…