A xorg-x11-server security update has been released for CentOS 6.

CentOS-announce: CESA-2020:4953 Important CentOS 6 xorg-x11-server Security Update

CentOS Errata and Security Advisory 2020:4953 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:4953

The following updated files have been uploaded and are currently
syncing to the mirrors: ( sha256sum Filename )

i386:
5059ab54170412872f88d5ab194293493b2b765f573fc6f78c0d9ace9f8d2b66 xorg-x11-server-common-1.17.4-18.el6.centos.i686.rpm
b37dff9ea4ac00896a290053c3144b2f8690f62e3b23ecac6180b02ec988a5cd xorg-x11-server-devel-1.17.4-18.el6.centos.i686.rpm
75a7add7ad8244095ad2e8700ed2c350aea28ed0e485d1f02e9e3482a3512b4d xorg-x11-server-source-1.17.4-18.el6.centos.noarch.rpm
77101c53e1cc3c2b6b305bf654160798a94a57a76ef9d78c387f82f89a2fb8c7 xorg-x11-server-Xdmx-1.17.4-18.el6.centos.i686.rpm
ac9a5141b765d2806881825618cc6e4d0e616598d06ecc299d475df3dd659840 xorg-x11-server-Xephyr-1.17.4-18.el6.centos.i686.rpm
9e064fdb6b18af8f4cc7cc899f39afcddf7998c4e8dd40aa3e39755bd038faca xorg-x11-server-Xnest-1.17.4-18.el6.centos.i686.rpm
60b79c706f5135af0aa04ba11e5c1285be3c8bddcbe41be35137c219b4624e88 xorg-x11-server-Xorg-1.17.4-18.el6.centos.i686.rpm
fee0a25a98d360e906683c7370f6a5a8b378ad29a257ae44d1cfde5dc6faccff xorg-x11-server-Xvfb-1.17.4-18.el6.centos.i686.rpm

x86_64:
e1d71fc00e3ba9364d4d54b30335f5444d9497f0d16bfed52f4b7bd3e9fd5e76 xorg-x11-server-common-1.17.4-18.el6.centos.x86_64.rpm
b37dff9ea4ac00896a290053c3144b2f8690f62e3b23ecac6180b02ec988a5cd xorg-x11-server-devel-1.17.4-18.el6.centos.i686.rpm
df8d9928f6cdd1cf9fcf0d5fba99f313385e47c6084056ddbf547b1b29e21c78 xorg-x11-server-devel-1.17.4-18.el6.centos.x86_64.rpm
75a7add7ad8244095ad2e8700ed2c350aea28ed0e485d1f02e9e3482a3512b4d xorg-x11-server-source-1.17.4-18.el6.centos.noarch.rpm
a9f913a126351611356eeb49a93d39dac03dd6e30b495c709027185f1f1459a7 xorg-x11-server-Xdmx-1.17.4-18.el6.centos.x86_64.rpm
9fcb2539a63002eada6c265e9bd1382847d7546f70ba619228933b89eb4d60e4 xorg-x11-server-Xephyr-1.17.4-18.el6.centos.x86_64.rpm
ccb1c58a72bd32faf005e31fb4a5a86bfebe7c9be9cc42095252f5a461dd968b xorg-x11-server-Xnest-1.17.4-18.el6.centos.x86_64.rpm
8f8fcc4e08eb21a9cbd85b98e6d77aab5562a0fc770a54d1cc479c000ff6d1cf xorg-x11-server-Xorg-1.17.4-18.el6.centos.x86_64.rpm
f9e253496ee6c40f20c21240a247cb8ad9d17f5be102ce8fc37da84e994e6b89 xorg-x11-server-Xvfb-1.17.4-18.el6.centos.x86_64.rpm

Source:
779305950f0a46d80e819083c60ea97e78b6fe9a71d55f29a3c9e67bf1c8ef58 xorg-x11-server-1.17.4-18.el6.centos.src.rpm