OpenComputers's SSRF to cloud service metadata services and local IPv6 addresses not blocked by default

Summary

OpenComputers is a Minecraft mod that adds programmable computers and robots to the game. This issue affects every version of OpenComputers with the Internet Card feature enabled; that is, OpenComputers 1.2.0 until 1.8.3 in their most common, default configurations. If the OpenComputers mod is installed as part of a Minecraft server hosted on a popular cloud hosting provider, such as AWS, GCP and Azure, those metadata services' API endpoints are not forbidden (aka "blacklisted") by default. As such, any player can gain access to sensitive information exposed via those metadata servers, potentially allowing them to pivot or privilege escalate into the hosting provider. In addition, IPv6 addresses are not correctly filtered at all, allowing broader access into the local IPv6 network. This can allow a player on a server using an OpenComputers computer to access parts of the private IPv4 address space, as well as the whole IPv6 address space, in order to retrieve sensitive information. OpenComputers v1.8.3 for Minecraft 1.7.10 and 1.12.2 contains a patch for this issue. Some workarounds are also available. One may disable the Internet Card feature completely. If using OpenComputers 1.3.0 or above, using the allow list (`opencomputers.internet.whitelist` option) will prohibit connections to any IP addresses and/or domains not listed; or one may add entries to the block list (`opencomputers.internet.blacklist` option). More information about mitigations is available in the GitHub Security Advisory.