Redis is an open source, in-memory database that persists on disk. An authenticated user may use a specially crafted Lua script to manipulate the garbage collector and potentially lead to remote code execution. The problem is fixed in 7.4.2, 7.2.7, and 6.2.17. An additional workaround to mitigate the problem without patching the redis-server executable is to prevent users from executing Lua scripts. This can be done using ACL to restrict EVAL and EVALSHA commands.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.