unable to locate maven clean plugin in my pom.xml

unable to locate maven clean plugin in my pom.xml

I am executing maven clean command in parallel mode.
I am getting following warning.
[WARNING] The following plugins are not marked @threadSafe in yourwebapp:
[WARNING] org.apache.maven.plugins:maven-clean-plugin:2.1.1
[WARNING] org.codehaus.mojo:cobertura-maven-plugin:2.6
I could find org.codehaus.mojo:cobertura-maven-plugin:2.6 plugin but
unable to find org.apache.maven.plugins:maven-clean-plugin in my pom.xml.
I know that I need to change the version of these plugin to a threadsafe
version. But Unable to locate them.
can some one help where to locate maven clean plugin.