kanassu@server1$ rpm -V yum
S.5....T.    /usr/bin/yum

This rpm -V yum output indicates that the size, cryptographic hash and modification time of your yum executable are not what they should be. It may have been damaged, or your system may have been hacked into and rootkitted. The fact that there are no error messages makes me worry that the later option might be true.

If you suspect your system has been hacked, see what to do if your system is compromised on Information Security StackExchange.

But if you are convinced it's simply damage, you should download from access.redhat.com (or copy from your installation media) a good version of yum-3.4.3-154.el7.noarch.rpm and use a command like

sudo rpm --reinstall /path/to/yum-3.4.3-154.el7.noarch.rpm

to repair it by reinstalling a good version over the bad one.

The easiest method by far is to make use of a binary build that's provided through a YUM repository. One such option would be to use the hop5.in repository. Specifically this page: gcc - Various compilers (C, C++, Objective-C, Java, ...). They're providing 4.8.2 which should work with CentOS 6.3 or 6.4. You might want to do an update prior:

$ sudo yum update

The other option would be to make use of the Developer Toolset, specifically the bundled version provided by Scientific Linux.

• http://linux.web.cern.ch/linux/devtoolset/

Following the installation instructions you'll basically do the following 2 steps:

$ sudo wget -O /etc/yum.repos.d/slc6-devtoolset.repo \
    http://linuxsoft.cern.ch/cern/devtoolset/slc6-devtoolset.repo
$ wget -O /etc/yum.repos.d/slc5-devtoolset.repo \
    http://linuxsoft.cern.ch/cern/devtoolset/slc5-devtoolset.repo

$ sudo yum install devtoolset-2

Update #1

The hop5.in YUM repository appears to have been removed, so the only recourse is to make use of the devtoolset method highlighted above.

Additional examples for installing via devtoolset are highlighted in this GitHub Gist: Installing gcc 4.8 and Linuxbrew on CentOS 6.