Simple Burp extension using performAction()

BurpExtender.kt

package burp

@Suppress("unused") // Remove IDE warning, as this class will be used by burp anyway
class BurpExtender : IBurpExtender, ISessionHandlingAction {

    private lateinit var cb : IBurpExtenderCallbacks

    override fun registerExtenderCallbacks(callbacks: IBurpExtenderCallbacks) {
        val extensionName = "A basic Kotlin template"

        // Expose callbacks
        this.cb = callbacks

        // Define the extension name
        cb.setExtensionName(extensionName)

        // Register as a session handling action
        cb.registerSessionHandlingAction(this)
    }

    override fun getActionName(): String {
        return("Update UA")
    }

    override fun performAction(currentRequest: IHttpRequestResponse, macroItems: Array<IHttpRequestResponse>?) {

        // Parse the request
        val reqInfo = cb.helpers.analyzeRequest(currentRequest)

        // Get the original
        val originalBody = currentRequest.request.copyOfRange(reqInfo.bodyOffset, currentRequest.request.size)
        val originalHeaders = reqInfo.headers

        // Define what to change
        val headerName = "User-Agent:"
        val marker = "userId = agarri_fr"

        // Iterate through headers
        val newHeaders = mutableListOf<String>()
        for (originalHeader in originalHeaders) {
            val newHeader =
                // Edit the targeted header
                if (originalHeader.startsWith(headerName)) { "$originalHeader - $marker" }
                // Simply copy other ones
                else { originalHeader }
            newHeaders.add(newHeader)
        }

        // May be useful when debugging
        // cb.printOutput(headers.joinToString())

        // Build the modified request
        currentRequest.request = cb.helpers.buildHttpMessage(newHeaders, originalBody)
    }
}