Link Preview JS extracts web links information. Prior to 4.0.1, the library did not check for IPv6 loopback attacks. There was also a DNS attack, where an address could be resolved into an internal IP. This could cause internal data leaks. This vulnerability is fixed in 4.0.1.
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.