Sentinel: [HIGH] Fix SSRF via DNS resolution

.jules/sentinel.md

@@ -39,3 +39,11 @@
 **Prevention:**
 1. Parse URLs and check hostnames against `localhost` and private IP ranges using `ipaddress` module.
 2. Enforce strict length limits on user inputs (e.g., profile IDs) to prevent resource exhaustion or buffer abuse.
+
+## 2025-01-24 - [SSRF via DNS Resolution]
+**Vulnerability:** The `validate_folder_url` function performed checks on the initial hostname but did not resolve domains to their IP addresses. This allowed attackers to bypass private IP blocklists by using a custom domain (e.g., `local.test`) that resolves to a private IP (e.g., `127.0.0.1`), leading to SSRF.
+**Learning:** Validating hostnames without DNS resolution is insufficient for SSRF protection. Attackers can easily map public domains to internal IPs (DNS Rebinding/Pinning).
+**Prevention:**
+1. Resolve domains to their IP addresses using `socket.getaddrinfo`.
+2. Validate all resolved IPs against a blocklist of private/loopback ranges.
+3. Fail closed if DNS resolution fails.

main.py

@@ -20,6 +20,7 @@
 import sys
 import time
 import re
+import socket
 import concurrent.futures
 import threading
 import ipaddress
@@ -209,8 +210,19 @@
                 log.warning(f"Skipping unsafe URL (private IP): {sanitize_for_log(url)}")
                 return False
         except ValueError:
-            # Not an IP literal, it's a domain.
-            pass
+            # Not an IP literal, it's a domain. Resolve to check for private IPs.
+            try:
+                # Resolve hostname to IPs
+                addr_infos = socket.getaddrinfo(hostname, None)
+                for family, kind, proto, canonname, sockaddr in addr_infos:
+                    ip_str = sockaddr[0]
+                    resolved_ip = ipaddress.ip_address(ip_str)
+                    if resolved_ip.is_private or resolved_ip.is_loopback:
+                        log.warning(f"Skipping unsafe URL (domain {sanitize_for_log(hostname)} resolves to private IP {resolved_ip}): {sanitize_for_log(url)}")
+                        return False
+            except socket.gaierror:
+                log.warning(f"Could not resolve hostname {sanitize_for_log(hostname)}, treating as unsafe.")
+                return False
 
     except Exception as e:
         log.warning(f"Failed to validate URL {sanitize_for_log(url)}: {e}")