🛡️ Sentinel: Secure socket directory creation to prevent pre-creation and symlink attacks

[minibota]

🔐 Issue: The daemon created the Unix socket in a shared /tmp directory (fallback) with a predictable name, allowing local attackers to pre-create the socket (DoS) or potentially hijack connections. The original fix attempt introduced a TOCTOU race condition.

🧠 Risk: A local attacker could prevent the agent from starting or potentially intercept execution reports.

🛠️ Fix:

• Changed socket path to use a private subdirectory based on EUID: /tmp/cc-agent-<euid>/cc-agent.sock.
• Implemented ensureSocketDir with robust security checks:
  • Unconditional MkdirAll to minimize TOCTOU window.
  • os.Lstat to detect symlinks.
  • verifySocketDirOwnership to ensure directory is owned by the current user (preventing "confused deputy").
  • Strict 0700 permission enforcement.

🧪 Tests:

• Added TestGetSocketPath_SecureFallback to verify path structure.
• Added TestEnsureSocketDir to verify permission enforcement.
• Added TestEnsureSocketDir_SymlinkAttack to verify symlink rejection.
• Verified existing tests pass.

This PR was created by an automated nightly security task.

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PR created automatically by Jules for task 12974733664033318369 started by @minibota

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