The vulnerability stems from unconditional unsafe implementations of Send/Sync traits for HConsed<T> as shown in the GitHub issue code snippet. These implementations lacked bounds requiring T: Send/Sync, violating Rust's safety guarantees. The explicit unsafe impl blocks in lib.rs (lines 354-355 in historical version) allowed sharing of interior-mutable types like Cell across threads, enabling data races. The high confidence comes from direct evidence in the provided issue description showing the problematic trait implementations and their consequences.