This vulnerability occurs when the web application is loading resources from the external server. like, https://securitymeter-vuln.com/load.php?url=http://secuirtymeter-readteam.com/somethingHere, the parameter URL is vulnerable to SSRF. If attacker changes it to localhost, he might be able to gather information about localhost exposed services or endpoints or even ports by performing XSPA (Cross-Site Port Attack) or even may lead to Privilege Escalation, some times an attacker will not be able to add localhost (127.0.0.1) In the URL parameter likely due to validation then an attacker will use the vulnerable server as an attack proxy to attack other servers.
Use whitelisting instead of blacklisting services/protocols/IP's
if web application is solely reliant on HTTPS or HTTP to initiate requests, permit only these URL schemas, By disabling unused URL schemas, you deny attackers the ability to utilize the application to carry out requests via potentially harmful schemas, including dict://, file:///, and gopher://.
Enable Authentication on Internal Services