Sparkle is a software update framework for macOS. Prior to version 2.9.2, "Autoupdate/SUBinaryDeltaApply.m" enforces "relativePath.pathComponents containsObject:@".."" and rejects writes whose immediate parent directory IS itself a symbolic link, but does not detect symlinks deeper in the relative path. "Autoupdate/SPUSparkleDeltaArchive.m"'s "extractItem:" will create symlinks in the destination tree from archive content (no ".." check on the symlink target), and a subsequent "Extract" item targeting "<symlink>/foo/bar" then escapes the destination tree via "fopen(path, "wb")" because the kernel resolves the intermediate symlink during the open call. This is a defense-in-depth issue: exploitation requires a maliciously-crafted ".delta" that passes EdDSA signature verification, i.e. EdDSA private-key compromise. With the AppInstaller running as root for system-domain installs, it gives the holder of a stolen signing key arbitrary file write at root level via the delta-apply path, which is a strictly broader primitive than the "drop-in replacement bundle" install they would otherwise have. Version 2.9.2 contains a patch for the issue.