hi i have getting security issues regarding

""returnUrl = apexpages.currentpage().getparameters().get('returnUrl');""

and

""pagereference p = new pagereference(returnUrl); 
      return p;""

in the following class.

Please solve this issue.

public with sharing class classexample {  

   public classexample()

{

         c= new contact(); 

         paramMap = apexpages.currentpage().getparameters();    

           list1 = new List<String>(paramMap.keySet());    

      for(integer i=0;i<list1.size();i++){     

          paramName = list1.get(i);

              paramValue = paramMap.get(paramName);      

         if(paramName != 'returnUrl' && paramName !='core.apexpages.devmode.url')

{

              c.put(paramName,paramValue);       

        }       

              }  

         returnUrl = apexpages.currentpage().getparameters().get('returnUrl'); 

    }

public contact c;

public string lastname{get;set;}

public string firstname{get;set;}

public string paramName{get;set;}

public string paramValue{get;set;}

public List<String> list1{get;set;}

public  map<string,string> paramMap{get;set;}

public  string returnUrl;      

    public pagereference autorun()

{            

      if(c.lastname!=null)

{

            insert c;    

    }     

   if(returnUrl!=null)

{

             pagereference p = new pagereference(returnUrl);

        p.setredirect(true);  

     return p;       

    }else

      {     

     return null;     

  }

             }   }