Errata patches for tmux have been released for OpenBSD 6.7 and 6.8. tmux has a stack overflow in CSI parsing.
Binary updates for the amd64, i386, and arm64 platforms are available via the syspatch utility. Source code patches can be found on the respective errata page: https://www.openbsd.org/errata67.html https://www.openbsd.org/errata68.html