[SEC-V-01] Bearer token compared with non-constant-time `==`

[intendednull]

Audit finding from #300 (commit 679f9fe)

Severity: low (informational; auth was out of scope for the validation/DoS pass — see SEC-A-01 for the medium-severity version)
Category: security / timing oracle
File: crates/agent/src/server.rs:235
Obvious fix: yes

Description

bearer_auth_middleware does if provided == expected_token { ... } — Rust's default &str equality is not constant-time, enabling a byte-wise timing oracle.

Impact / Threat

Network-adjacent attacker can mount a timing oracle to recover the agent's MCP bearer token character by character.

Suggested fix

Compare via subtle::ConstantTimeEq::ct_eq(provided.as_bytes(), expected_token.as_bytes()), or hash both sides and compare fixed-length digests.

Verify

rg "provided == expected_token" crates/agent/src/server.rs