🛡️ Sentinel: [HIGH] Fix TOCTOU race condition in signature verifier nonce checking

[blue4209211]

Description

• 🚨 Severity: HIGH (Replay Protection Bypass)
• 💡 Vulnerability: In pkg/signing/verify.go, nonces were checked with isReplayedNonce() prior to signature verification, and recorded with recordNonce() only at the end of Verify().
• 🎯 Impact: Under concurrent load, multiple identical signed requests could execute isReplayedNonce() simultaneously before any goroutine recorded the nonce, allowing attackers to bypass replay attack protection and execute duplicate signed commands on datasources.
• 🔧 Fix:
  1. Updated pkg/signing/verify.go to perform cryptographic Ed25519 signature and payload integrity verification first.
  2. Replaced separate isReplayedNonce and recordNonce calls with an atomic checkAndRecordNonce(nonce) method under mutex lock v.nonceMu.
  3. Added TestVerify_ConcurrentReplay in pkg/signing/verify_test.go to test concurrent replay protection under 20 concurrent goroutines.
  4. Added critical security learning entry to .jules/sentinel.md.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which improves existing functionality)
• Refactor (non-breaking change which improves code structure)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation
• CI/CD

How Has This Been Tested?

• Unit tests
• Manual testing

Checklist

• Commits are signed off (git commit -s) per DCO
• make validate passes (fmt + lint + test)
• Docs updated if the wire shape, config surface, or proxy module behavior changed

[gemini-code-assist]

Code Review

This pull request resolves a TOCTOU race condition in the signature verification process by combining the nonce check and record operations into a single atomic method, and adds a concurrent replay test to verify the fix. The review feedback identifies a performance bottleneck in the nonce eviction logic under high concurrent load and suggests rate-limiting the eviction process to prevent latency spikes.