Methods, systems, and apparatus for use in a distributed client-side user monitoring and attack system are disclosed herein. An example method includes providing a first set of instructions from a security application server to a target application server, the first set of instructions to, when executed, cause a client device to transmit a request for an image to the security application server. In response to the request for the image, a connection is opened between the client device and the security application server. Via the connection opened in response to the request for the image, a second set of instructions is provided to cause the client device to perform a vulnerability test on the target application server and communicate a result of the vulnerability test via the connection.