An in-memory data store used for caching, session storage, job queues and rate limiting. It is fast because it keeps everything in memory, and it is commonly deployed as a supporting service that the application talks to over the local network.
Historically shipped with no authentication and bound to all interfaces, a combination that led to mass compromise — an exposed Redis can often be turned into remote code execution. Bind to localhost, set a password, and keep protected mode on.
An exposed Redis is frequently escalated to remote code execution rather than merely data theft. Close it, enable protected mode, set a password, and check for unexpected keys or scheduled tasks.
The port scanner will tell you whether 6379 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.