[SECURITY-420] RequirePOST for proxy validation

core/src/main/java/hudson/ProxyConfiguration.java

@@ -56,6 +56,7 @@ import org.apache.commons.httpclient.methods.GetMethod;
 import org.jvnet.robust_http_client.RetryableHttpStream;
 import org.kohsuke.stapler.DataBoundConstructor;
 import org.kohsuke.stapler.QueryParameter;
+import org.kohsuke.stapler.interceptor.RequirePOST;
 
 /**
  * HTTP proxy configuration.
@@ -296,6 +297,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
             return FormValidation.ok();
         }
 
+        @RequirePOST
         public FormValidation doValidateProxy(
                 @QueryParameter("testUrl") String testUrl, @QueryParameter("name") String name, @QueryParameter("port") int port,
                 @QueryParameter("userName") String userName, @QueryParameter("password") String password,