The vulnerability stems from the unsafe Sync trait implementation for Intern<T> that lacks a T: Sync bound. This allows Intern<T> to be considered thread-safe even when containing non-Sync types, violating Rust's thread safety guarantees. The GitHub issue reproduction demonstrates this by creating a data race with Cell<T> (which is !Sync) stored in Intern<T>. The fix in 0.4.2 added the T: Sync bound to this implementation, confirming this was the root cause.