If kdcproxy receives a request for a realm which does not have server addresses defined in its configuration, by default, it will query SRV records in the DNS zone matching the requested realm name. This creates a server-side request forgery vulnerability, since an attacker could send a request for a realm matching a DNS zone where they created SRV records pointing to arbitrary ports and hostnames (which may resolve to loopback or internal IP addresses). This vulnerability can be exploited to probe internal network topology and firewall rules, perform port scanning, and exfiltrate data. Deployments where the use_dns setting is explicitly set to false are not affected.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.