A telnet security update has been released for CentOS 6.

CentOS-announce: CESA-2020:1335 Important CentOS 6 telnet Security Update

CentOS Errata and Security Advisory 2020:1335 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:1335

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

i386:
923d72923ba2cd24753cc70ac63cf25d24718d3c228f6f92d2254ec7d73ef590 telnet-0.17-49.el6_10.i686.rpm
63af65efc0cf601f982af8bc595e12980d43b3c4b9872a273b518d05057c07b6 telnet-server-0.17-49.el6_10.i686.rpm

x86_64:
591ba5f1a15b7b68106ddd4021fb4ff5f9c405535efe3b72375fec756ac414c9 telnet-0.17-49.el6_10.x86_64.rpm
4fb370c991172abffe095b182b70b2ed7badea2ae9db89e33c28d66b5c9c339c telnet-server-0.17-49.el6_10.x86_64.rpm

Source:
7aba1d5e792fe1bbf45000f404f712727a55724523c36c88f26abe3c0cd1e91b telnet-0.17-49.el6_10.src.rpm