zkLogin: when ZKP is not enough

Security researchers at Brave identified critical vulnerabilities in zkLogin, a zero-knowledge proof-based authorization system used in the Sui blockchain ecosystem. The analysis reveals three major vulnerability classes: ambiguous JWT parsing that allows claim shadowing and parser differentials, weak binding between authentication and authorization enabling cross-RP impersonation attacks, and centralization risks from outsourced proving services. The core issue is that zkLogin's security depends not just on cryptographic proofs, but on external assumptions about document correctness, issuer governance, and execution environments that the protocol doesn't enforce. The research demonstrates that repurposing short-lived OIDC tokens into long-lived authorization credentials without proper validation, canonical parsing, and cryptographic binding creates exploitable attack surfaces even when the underlying ZKP cryptography is sound.