Vim is an open source command line text editor. When closing a window, vim may try to access already freed window structure. Exploitation beyond crashing the application has not been shown to be viable. This issue has been addressed in commit `25aabc2b` which has been included in release version 9.0.2106. Users are advised to upgrade. There are no known workarounds for this vulnerability.
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 Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.