A libX11 security update has been released for CentOS 6.

CentOS-announce: CESA-2020:4946 Important CentOS 6 libX11 Security Update

CentOS Errata and Security Advisory 2020:4946 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:4946

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

i386:
2ac64371cfe0b0675ac6e0d137996d04dde05bdc4fb74f049e6f18c684d0c54c libX11-1.6.4-4.el6_10.i686.rpm
42982c0e189057e646b9af3fad89c9590a25107015b11f1d294d7cc087be2d07 libX11-common-1.6.4-4.el6_10.noarch.rpm
d1efeb7df384c9de08034a296df0f3b229c75d73b20e97ac95d583ccbf459660 libX11-devel-1.6.4-4.el6_10.i686.rpm

x86_64:
2ac64371cfe0b0675ac6e0d137996d04dde05bdc4fb74f049e6f18c684d0c54c libX11-1.6.4-4.el6_10.i686.rpm
4f8aab32eae845104e60b88ad0d42025d7b797215af410e56445ec7615fb1142 libX11-1.6.4-4.el6_10.x86_64.rpm
42982c0e189057e646b9af3fad89c9590a25107015b11f1d294d7cc087be2d07 libX11-common-1.6.4-4.el6_10.noarch.rpm
d1efeb7df384c9de08034a296df0f3b229c75d73b20e97ac95d583ccbf459660 libX11-devel-1.6.4-4.el6_10.i686.rpm
128019277a9c9de7c62b5c929e63e2e8590c0f4f07264e7efe12f1ce69acbcc9 libX11-devel-1.6.4-4.el6_10.x86_64.rpm

Source:
5ec3db37efff7035c80185164dfb021cc61a3350a98e5229bc4b7b035093e8bb libX11-1.6.4-4.el6_10.src.rpm