untrusted comment: verify with openbsd-72-base.pub
RWQTKNnK3CZZ8JJprv7fn3dHGG/kYxEmVpLQud7vt6ZmE2J4gG0X2UpLfpOBdj4npwJkrn7Sj31Vb40C0ka7qSU5/ouguk1Lig8=

OpenBSD 7.2 errata 027, June 15, 2023:

libX11 CVE-2023-3138 Missing checks in XQueryExtension() return values.

Apply by doing:
    signify -Vep /etc/signify/openbsd-72-base.pub -x 027_libx11.patch.sig \
        -m - | (cd /usr/xenocara && patch -p0)

And then rebuild and install libX11:
    cd /usr/xenocara/lib/libX11
    make -f Makefile.bsd-wrapper obj
    make -f Makefile.bsd-wrapper build

Index: lib/libX11/src/InitExt.c
===================================================================
RCS file: /cvs/xenocara/lib/libX11/src/InitExt.c,v
diff -u -p -r1.7 InitExt.c
--- lib/libX11/src/InitExt.c	6 Apr 2015 20:57:59 -0000	1.7
+++ lib/libX11/src/InitExt.c	12 Jun 2023 23:19:11 -0000
@@ -33,6 +33,18 @@ from The Open Group.
 #include <X11/Xos.h>
 #include <stdio.h>
 
+/* The X11 protocol spec reserves events 64 through 127 for extensions */
+#ifndef LastExtensionEvent
+#define LastExtensionEvent 127
+#endif
+
+/* The X11 protocol spec reserves requests 128 through 255 for extensions */
+#ifndef LastExtensionRequest
+#define FirstExtensionRequest 128
+#define LastExtensionRequest 255
+#endif
+
+
 /*
  * This routine is used to link a extension in so it will be called
  * at appropriate times.
@@ -242,6 +254,12 @@ WireToEventType XESetWireToEvent(
 	WireToEventType proc)	/* routine to call when converting event */
 {
 	register WireToEventType oldproc;
+	if (event_number < 0 ||
+	    event_number > LastExtensionEvent) {
+	    fprintf(stderr, "Xlib: ignoring invalid extension event %d\n",
+		    event_number);
+	    return (WireToEventType)_XUnknownWireEvent;
+	}
 	if (proc == NULL) proc = (WireToEventType)_XUnknownWireEvent;
 	LockDisplay (dpy);
 	oldproc = dpy->event_vec[event_number];
@@ -263,6 +281,12 @@ WireToEventCookieType XESetWireToEventCo
     )
 {
 	WireToEventCookieType oldproc;
+	if (extension < FirstExtensionRequest ||
+	    extension > LastExtensionRequest) {
+	    fprintf(stderr, "Xlib: ignoring invalid extension opcode %d\n",
+		    extension);
+	    return (WireToEventCookieType)_XUnknownWireEventCookie;
+	}
 	if (proc == NULL) proc = (WireToEventCookieType)_XUnknownWireEventCookie;
 	LockDisplay (dpy);
 	oldproc = dpy->generic_event_vec[extension & 0x7F];
@@ -284,6 +308,12 @@ CopyEventCookieType XESetCopyEventCookie
     )
 {
 	CopyEventCookieType oldproc;
+	if (extension < FirstExtensionRequest ||
+	    extension > LastExtensionRequest) {
+	    fprintf(stderr, "Xlib: ignoring invalid extension opcode %d\n",
+		    extension);
+	    return (CopyEventCookieType)_XUnknownCopyEventCookie;
+	}
 	if (proc == NULL) proc = (CopyEventCookieType)_XUnknownCopyEventCookie;
 	LockDisplay (dpy);
 	oldproc = dpy->generic_event_copy_vec[extension & 0x7F];
@@ -305,6 +335,12 @@ EventToWireType XESetEventToWire(
 	EventToWireType proc)	/* routine to call when converting event */
 {
 	register EventToWireType oldproc;
+	if (event_number < 0 ||
+	    event_number > LastExtensionEvent) {
+	    fprintf(stderr, "Xlib: ignoring invalid extension event %d\n",
+		    event_number);
+	    return (EventToWireType)_XUnknownNativeEvent;
+	}
 	if (proc == NULL) proc = (EventToWireType) _XUnknownNativeEvent;
 	LockDisplay (dpy);
 	oldproc = dpy->wire_vec[event_number];
@@ -325,6 +361,12 @@ WireToErrorType XESetWireToError(
 	WireToErrorType proc)	/* routine to call when converting error */
 {
 	register WireToErrorType oldproc = NULL;
+	if (error_number < 0 ||
+	    error_number > LastExtensionError) {
+	   fprintf(stderr, "Xlib: ignoring invalid extension error %d\n",
+		    error_number);
+	   return (WireToErrorType)_XDefaultWireError;
+	}
 	if (proc == NULL) proc = (WireToErrorType)_XDefaultWireError;
 	LockDisplay (dpy);
 	if (!dpy->error_vec) {