Web Vulnerabilities
Cloud Metadata Endpoint
Also: instance metadata service, IMDS
A special internal address inside cloud servers that hands out details about the machine, sometimes including temporary credentials.
In more detail
Cloud providers expose this service only to code running on the machine, at a well-known link-local address. A server-side request forgery bug is dangerous because the server can be talked into asking that address on an outsider's behalf. Newer versions of the services add protections, and servers should be configured to require them.