Errata patches for Mesa have been released for OpenBSD 6.5 and 6.6. Shared memory regions used by some Mesa drivers had permissions which allowed others to access that memory.
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/errata65.html https://www.openbsd.org/errata66.html
