🛡️ Sentinel: [MEDIUM] Fix missing input length limits

[arumes31]

🚨 Severity: MEDIUM

💡 Vulnerability: The application extracted fields from HTTP requests using c.PostForm() without verifying the length of the provided strings. This allowed an attacker to submit massive payloads (e.g., a 2GB username), which the server would allocate memory for during processing, leading to rapid memory exhaustion and Denial of Service (CWE-400).

🎯 Impact: An unauthenticated or authenticated attacker could crash the server by continuously submitting requests with excessively large form fields, severely impacting availability.

🔧 Fix: Added defensive bounds checking (e.g., len(field) > MAX_LEN) immediately after data extraction in auth_handlers.go, excluded_handlers.go, and ip_handlers.go. Short fields (usernames, TOTP codes) are capped at 255 bytes, while longer fields (passwords, allowed IPs, URLs) are capped at 2048 bytes. Exceeding these limits securely rejects the request.

✅ Verification:

1. Execute go test -short ./... to ensure no functionality is broken.
2. Attempt to login with an overly long username (e.g., curl -d "username=$(head -c 300 < /dev/zero | tr '\0' 'A')&password=foo" -X POST http://localhost:8080/login) and verify it fails securely with an "Invalid input length" error rather than processing it.

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PR created automatically by Jules for task 1616583081484492151 started by @arumes31

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