Errata patches for libX11 have been released for OpenBSD 6.6 and 6.7. An integer overflow in libX11 could lead to a double free.
Additionally, fix a regression in ximcp. 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/errata66.html https://www.openbsd.org/errata67.html