🛡️ Sentinel: [HIGH] Fix timing attack and info disclosure in internal router

[ivanleekk]

🚨 Severity: HIGH
💡 Vulnerability: The internal scheduler API (/internal/tasks/daily-snapshot) had two security issues:

1. It used a standard string equality check (!=) for a secret token in a custom HTTP Header, opening it up to timing attacks.
2. The endpoint’s generic exception handler returned the raw exception string (str(e)) in a 500 error response, potentially leaking internal implementation details (e.g., database connection strings, file paths) to the client.

🎯 Impact:

• An attacker could theoretically exploit the timing attack to guess the SCHEDULER_SECRET character by character.
• The information disclosure could aid an attacker by revealing sensitive system internals upon triggering unexpected errors.

🔧 Fix:

• Replaced the != check with secrets.compare_digest for constant-time comparison to prevent timing attacks, adding an explicit is None check to prevent runtime type errors if the header is omitted.
• Updated the generic exception handler to return a standard "An internal server error occurred" message to the client, while utilizing logger.error("...", exc_info=True) to internally record the stack trace and maintain observability.

✅ Verification:

• The modifications in backend/src/routers/internal.py have been verified.
• The Python code has been successfully linted (uv run ruff check src/routers/internal.py).

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PR created automatically by Jules for task 16789968966607745194 started by @ivanleekk

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