Race Condition in Audit Logger Exposes Hash Chain Integrity Flaw — 14 Breaks Detected in Multi-Agent Sessions

A race condition in the audit logging subsystem has been identified as the source of hash chain integrity failures across multiple audit files. During end-to-end internal testing, automated diagnostics flagged 14 distinct chain breaks in audit log files spanning from April 11 to May 6, 2026. The root cause lies in how concurrent writes from multiple agent sessions interact with the chained hash mechanism that secures each audit record.

The audit logger relies on a hash chain where each new record must reference the previous record's hash via a `prevHash` field. When separate agent sessions write audit records simultaneously, the system becomes vulnerable to a classic time-of-check-time-of-use (TOCTOU) race condition. Two sessions can read the same chain tip, retrieve identical `prevHash` values, and then write records that reference the same parent—creating parallel branches that the verification logic correctly identifies as orphan chains. Evidence shows the expected and actual hashes consistently differ by exactly one record, a hallmark pattern of concurrent write interference. The breaks concentrate during periods of elevated multi-agent activity, corroborating the concurrency hypothesis.

The vulnerability raises concerns about the reliability of audit trail verification in distributed or multi-session environments. If left unaddressed, the integrity mechanism designed to detect tampering could itself produce false negatives when parallel writes corrupt the chain structure. The underlying issue is structural rather than algorithmic—the hash computation itself is sound, but the write serialization layer fails to enforce atomic chain-tip updates. Patching will require introducing synchronization around the read-write cycle of the chain tip, either through file locking, a write-ahead log, or a dedicated serialization queue for audit record ingestion.