Agent WranglersRequests and files Required
Fetch only allowed addresses
When the server fetches an address a user gave, allow only the hosts it needs. Never reach internal or cloud metadata addresses.
How to do it
Keep an allow-list of hosts. Resolve the name, and refuse private, loopback and link-local addresses, in IPv4 and IPv6. Connect to the address you checked, so a second DNS answer can't swap it. Check every redirect again, or turn redirects off. Use a library or an egress proxy rather than your own IP parser. On AWS, require IMDSv2.
Example
http://127.0.0.1:8080/
http://169.254.169.254/latest/meta-data/
http://[::1]/
http://0x7f000001/
http://0.0.0.0:8080/
http://metadata.google.internal/computeMetadata/v1/
http://[fd00:ec2::254]/latest/meta-data/
https://redirect.example.net/to?url=http://10.0.0.5/How to check
The agent gives the fetch feature internal addresses, including one behind a redirect. Every one is refused.
For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers security testing playbook: https://agentwranglers.se/playbook/adversarial-testing/requests-and-files/block-server-side-request-forgery.