security(acp): use constant-time comparison for ACP auth token validation to prevent timing side-channel #2448
Description
Gap Source
Goose v1.28.0 (March 18, 2026) added constant-time token comparison. Competitive parity scan CI-307.
What Is Missing
Zeph's ACP HTTP transport auth uses standard string equality (==) for bearer token validation. Standard string comparison short-circuits on first mismatch — a timing oracle that allows remote attackers to enumerate valid token prefixes.
Fix
Replace string equality with constant-time comparison using the subtle crate (ConstantTimeEq) or ring::constant_time::verify_slices_are_equal.
Location: ACP HTTP auth middleware in crates/zeph-acp/src/ (HTTP transport bearer validation).
Priority
P2 — security correctness. One-line fix with the subtle crate. ACP HTTP transport ships in production builds — this is an exploitable side-channel for any deployment with bearer auth enabled.