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Last updated: May 08, 2023

Testing WebViews Cleanup


To test for WebViews cleanup you should inspect all APIs related to WebView data deletion and try to fully track the data deletion process.

Static Analysis

Start by identifying the usage of the following WebView APIs and carefully validate the mentioned best practices.

  • Initialization: an app might be initializing the WebView in a way to avoid storing certain information by using setDomStorageEnabled, setAppCacheEnabled or setDatabaseEnabled from android.webkit.WebSettings. The DOM Storage (for using the HTML5 local storage), Application Caches and Database Storage APIs are disabled by default, but apps might set these settings explicitly to "true".

  • Cache: Android's WebView class offers the clearCache method which can be used to clear the cache for all WebViews used by the app. It receives a boolean input parameter (includeDiskFiles) which will wipe all stored resource including the RAM cache. However if it's set to false, it will only clear the RAM cache. Check the app for usage of the clearCache method and verify its input parameter. Additionally, you may also check if the app is overriding onRenderProcessUnresponsive for the case when the WebView might become unresponsive, as the clearCache method might also be called from there.

  • WebStorage APIs: WebStorage.deleteAllData can be also used to clear all storage currently being used by the JavaScript storage APIs, including the Web SQL Database and the HTML5 Web Storage APIs.

    Some apps will need to enable the DOM storage in order to display some HTML5 sites that use local storage. This should be carefully investigated as this might contain sensitive data.

  • Cookies: any existing cookies can be deleted by using CookieManager.removeAllCookies.

  • File APIs: proper data deletion in certain directories might not be that straightforward, some apps use a pragmatic solution which is to manually delete selected directories known to hold user data. This can be done using the API such as


This example in Kotlin from the open source Firefox Focus app shows different cleanup steps:

override fun cleanup() {
    clearFormData() // Removes the autocomplete popup from the currently focused form field, if present. Note this only affects the display of the autocomplete popup, it does not remove any saved form data from this WebView's store. To do that, use WebViewDatabase#clearFormData.


    WebStorage.getInstance().deleteAllData() // Clears all storage currently being used by the JavaScript storage APIs. This includes the Application Cache, Web SQL Database and the HTML5 Web Storage APIs.

    val webViewDatabase = WebViewDatabase.getInstance(context)
    // It isn't entirely clear how this differs from WebView.clearFormData()
    webViewDatabase.clearFormData() // Clears any saved data for web forms.

    deleteContentFromKnownLocations(context) // calls FileUtils.deleteWebViewDirectory(context) which deletes all content in "app_webview".

The function finishes with some extra manual file deletion in deleteContentFromKnownLocations which calls functions from FileUtils. These functions use the method to recursively delete files from the specified directories.

private fun deleteContent(directory: File, doNotEraseWhitelist: Set<String> = emptySet()): Boolean {
    val filesToDelete = directory.listFiles()?.filter { !doNotEraseWhitelist.contains( } ?: return false
    return filesToDelete.all { it.deleteRecursively() }

Dynamic Analysis

Open a WebView accessing sensitive data and then log out of the application. Access the application's storage container and make sure all WebView related files are deleted. The following files and folders are typically related to WebViews:

  • app_webview
  • Cookies
  • pref_store
  • blob_storage
  • Session Storage
  • Web Data
  • Service Worker