🛡️ Sentinel: [CRITICAL] Fix Path Traversal in Auth Files API

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: Cross-platform path traversal vulnerability. The UploadAuthFile and DeleteAuthFile APIs relied on strings.Contains(name, string(os.PathSeparator)) to sanitize filenames. On Windows, os.PathSeparator is \ meaning / is permitted, allowing an attacker to supply a payload like ../../secrets/file.json and traverse directories. In UploadAuthFile, file.Filename wasn't explicitly verified.
🎯 Impact: Malicious actors could delete or replace arbitrary .json files outside the designated AuthDir directory on Windows hosts if the API relies on local paths, potentially compromising internal keys and data or triggering Denial of Service (DoS) by wiping configuration.
🔧 Fix: Replaced strings.Contains with strings.ContainsAny(name, "/\\") for both queries and form payloads. Validates file.Filename explicitly alongside the query params.
✅ Verification: Verified by existing security unit tests (auth_files_security_test.go) and standard go test ./... -short which are confirmed to pass properly with this security check.

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PR created automatically by Jules for task 8781413356012186276 started by @rschumann

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