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

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 on its <bundleId>-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.