GHSA-g3hp-f6mg-559v Sparkle's AppInstaller post-stage-1 XPC listener accepts unvalidated connections, allowing spoofed appcast item data injection

• SwiftURL github.com/sparkle-project/Sparkle: <= 2.9.1 (no fix yet)

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 `-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 `-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` on its `-spks` status service, and the launching app displays attacker-controlled release notes (name, version, critical flag). Note: Sparkle can be used to update other app bundles, so the "app doing the updating" and the "app being updated" are not necessarily the same bundle. In the system-domain case (`SPUUsesSystemDomainForBundlePath = true`), the AppInstaller runs as root via `SMJobSubmit` to `kSMDomainSystemLaunchd`, and the Mach service is reachable by any local user process. Affected versions: 2.x branch including 2.9.1. Impact A local user-level process can inject a forged `SUAppcastItem` (arbitrary name, version, critical flag) into the progress agent's status broadcast. Other Sparkle-aware clients on the system will display attacker-controlled release notes as authoritative installation state. The integrity of the installed code is not affected - the bundle moved into place is the legitimate, signature-validated update from stage 1. The impact is limited to UI spoofing of installation metadata. Remediation Enforce `SUCodeSigningVerifier validateConnection:` on all new connections regardless of installation stage, or disallow `SPUSentUpdateAppcastItemData` after the active connection invalidates.