Got this today in my inbox:

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X.Org Security Advisory: June 15, 2023

Buffer overflows in InitExt.c in libX11 prior to 1.8.6 [CVE-2023-3138]
====================================================

The functions in src/InitExt.c in libX11 prior to 1.8.6 do not check
that the values provided for the Request, Event, or Error IDs are
within the bounds of the arrays that those functions write to, using
those IDs as array indexes.  Instead they trusted that they were called
with values provided by an Xserver that was adhering to the bounds
specified in the X11 protocol, as all X servers provided by X.Org do.

As the protocol only specifies a single byte for these values, an
out-of-bounds value provided by a malicious server (or a malicious
proxy-in-the-middle) can only overwrite other portions of the Display
structure and not write outside the bounds of the Display structure
itself.  Testing has found it is possible to at least cause the client
to crash with this memory corruption.

This is fixed in:
https://gitlab.freedesktop.org/xorg/lib … 0332cfa36c
which is included in the libX11 1.8.6 release issued today.

X.Org thanks Gregory James Duck for reporting this issue to our security team.

--
Alan Coopersmith - alan.coopersmith@oracle.com
X.Org Security Response Team - xorg-security@lists.x.org

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Best,

A.