The vulnerability stemmed from improper validation of socket filenames reported by application processes during startup. The pre-patch version of handleSpawnResponse directly processed socket addresses from untrusted sources without:

1. Checking if paths were absolute
2. Verifying file ownership
3. Preventing directory traversal (../ patterns)

This allowed malicious applications to report crafted socket paths that would get processed by Passenger's cleanup routines, leading to arbitrary file deletion. The commit added validateSocketAddress checks and path validation logic to fix this.