Land rapid7#7499, check method for pkexec exploit

Author: wvu

modules/exploits/linux/local/pkexec.rb

@@ -66,6 +66,24 @@ def executable_path
     @executable_path
   end
 
+  def check
+    # version can be nil
+    version = cmd_exec('pkexec --version').split.last
+
+    # version can be a string, so we check it
+    if version.nil? || !Gem::Version.correct?(version)
+      vprint_error('pkexec not found or version incorrect')
+      return CheckCode::Unknown
+    end
+
+    if Gem::Version.new(version) <= Gem::Version.new('0.101')
+      vprint_good("pkexec #{version} found")
+      return CheckCode::Appears
+    end
+
+    CheckCode::Detected
+  end
+
   def exploit
     main = %q^
 /*