The previous blocks of code showed all of the extension content and the graphical interface, but the following lines show the operation of the extension itself:

First, we set up the initial variables and components, as follows:

  @Override 
  public String getTabCaption() { return "Burp UserAgent"; } 
 
  @Override 
  public Component getUiComponent() { return bUAPanel; } 
   
  @Override 
  public String getActionName(){ return "Burp UserAgent"; } 
   
  @Override 
  public void performAction(IHttpRequestResponse currentRequest, IHttpRequestResponse[] macroItems) { 
       
    IRequestInfo requestInfo = extHelpers.analyzeRequest(currentRequest); 
    List<String> headers = requestInfo.getHeaders(); 
    String reqRaw = new String(currentRequest.getRequest()); 
    String reqBody = reqRaw.substring(requestInfo.getBodyOffset()); 
    Integer uaInHeader = 0; 
 
    if (!newUA.startsWith("Current Browser")) { 
         
      for (int i = 0; i < headers.size(); i++) { 

The following code is the main part of the extension. Here, a loop is created to add the values, that is, to substitute the user-agents:

        if (headers.get(i).startsWith("User-Agent:") && !headers.get(i).startsWith("User-Agent: " + newUA)) { 
          headers.set(i, "User-Agent: " + newUA); 
          uaInHeader = 1; 
        } else if (headers.get(i).startsWith("User-Agent: " + newUA)) { 
          uaInHeader = 1; 
        } 
      } 
    } 
     
    if (uaInHeader == 0 && !newUA.startsWith("Current Browser")) { 
        headers.add("User-Agent: " + newUA); 
    } 
     
    byte[] message = extHelpers.buildHttpMessage(headers, reqBody.getBytes()); 
 
    currentRequest.setRequest(message); 
  } 
} 

Now that we are done writing the extension, let's go ahead and execute it.