🛡️ Sentinel: [CRITICAL] Fix Server-Side Request Forgery (SSRF) vulnerabilities

[himattm]

🚨 Severity: CRITICAL
💡 Vulnerability: internal/plugin/manager.go used http.Get() to fetch plugins from untrusted, user-provided URLs without restricting the resolved IP addresses, leading to potential Server-Side Request Forgery (SSRF). This could allow an attacker to bypass firewalls and access restricted internal networks, such as cloud metadata endpoints (e.g., 169.254.169.254).
🎯 Impact: Attackers could potentially extract sensitive credentials or enumerate internal infrastructure by tricking the application into making unauthorized HTTP requests.
🔧 Fix: Replaced http.Get() with a custom http.Client initialized via newSafeClient(). This client utilizes a net.Dialer with a Control hook that evaluates the resolved IP address after DNS resolution but before the socket connection is made, effectively blocking restricted IPs like 169.254.169.254 and preventing DNS rebinding attacks. Also disabled KeepAlives on the custom transport since these clients are one-off.
✅ Verification: Ensure the test suite passes (go test ./...) and that attempting to add a plugin from a prohibited IP (e.g., http://169.254.169.254/plugin.sh) fails with the intended forbidden error.

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PR created automatically by Jules for task 10951292170685207095 started by @himattm

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