Vim is a UNIX editor that, prior to version 9.0.2121, has a heap-use-after-free vulnerability. When executing a `:s` command for the very first time and using a sub-replace-special atom inside the substitution part, it is possible that the recursive `:s` call causes free-ing of memory which may later then be accessed by the initial `:s` command. The user must intentionally execute the payload and the whole process is a bit tricky to do since it seems to work only reliably for the very first :s command. It may also cause a crash of Vim. Version 9.0.2121 contains a fix for this issue.
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 Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.