RSSHub vulnerable to SSRF in /mastodon, /zjoi, and /m4

RSSHub is an open source RSS feed generator. Prior to version 1.0.0-master.a429472, RSSHub allows remote attackers to use the server as a proxy to send HTTP GET requests to arbitrary targets and retrieve information in the internal network or conduct Denial-of-Service (DoS) attacks. The attacker can send malicious requests to a RSSHub server, to make the server send HTTP GET requests to arbitrary destinations and see partial responses. This may lead to leak the server IP address, which could be hidden behind a CDN; retrieving information in the internal network, e.g. which addresses/ports are accessible, the titles and meta descriptions of HTML pages; and denial of service amplification. The attacker could request the server to download some large files, or chain several SSRF requests in a single attacker request.