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WebResourceResponse Vulnerabilities

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WebResourceResponse overview

WebResourceResponsearrow-up-right is a class that allows an Android application to emulate the server within WebView by intercepting requests and returning arbitrary content (including a status code, content type, content encoding, headers and the response body) from the application's code itself without making any actual requests to the server.

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Security issues

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Access to arbitrary files

If you control a path of the returned file and have a XSS or the ability to open arbitrary links inside a WebView, you can gain access to arbitrary files via XHR requests.

For example, if there is the following WebResourceResponse implementation:

WebView webView = findViewById(R.id.webView);
webView.setWebViewClient(new WebViewClient() {
   public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
       Uri uri = request.getUrl();
       if (uri.getPath().startsWith("/local_cache/")) {
           File cacheFile = new File(getCacheDir(), uri.getLastPathSegment());
           if (cacheFile.exists()) {
               InputStream inputStream;
               try {
                   inputStream = new FileInputStream(cacheFile);
               } catch (IOException e) {
                   return null;
               }
               Map<String, String> headers = new HashMap<>();
               headers.put("Access-Control-Allow-Origin", "*");
               return new WebResourceResponse("text/html", "utf-8", 200, "OK", headers, inputStream);
           }
       }
       return super.shouldInterceptRequest(view, request);
   }
});

The PoC for the attack may look like the following one:

In the above example, the attack is possible because the Uri.getLastPathSegment() returns a decoded value that is used to generate the file path within the new File(getCacheDir(), uri.getLastPathSegment()) line.

Policies like CORS still work inside a WebView. Therefore, requests to the any.domain are not allowed without the Access-Control-Allow-Origin: * header. However, this restriction does not affect this PoC since the WebResourceResponse implementation checks uses only the URL path and you can replace any.domain with the current origin.

References:

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References

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