AG-50239 Fix 'prevent-fetch' — do not use 'modifyResponse' on redirected requests. #545

Squashed commit of the following:

commit e0ac5b3
Author: Adam Wróblewski <adam@adguard.com>
Date:   Fri Feb 27 11:37:41 2026 +0100

    Add newline

commit 7602669
Author: Adam Wróblewski <adam@adguard.com>
Date:   Fri Feb 27 11:30:31 2026 +0100

    Add extension scheme check and related tests to prevent-fetch scriptlet

Author: AdamWr

CHANGELOG.md

@@ -21,13 +21,15 @@ The format is based on [Keep a Changelog], and this project adheres to [Semantic
 
 ### Fixed
 
+- `prevent-fetch` — do not use `modifyResponse` on redirected requests [#545].
 - issue with assigning `window` to property which was not set in
   `abort-on-property-read`, `abort-on-property-write`, `abort-on-stack-trace`,
   `abort-current-inline-script`, `debug-on-property-write`, `debug-on-property-read`,
   `debug-current-inline-script` and `log-on-stack-trace` scriptlets [#513].
 
 [Unreleased]: https://github.com/AdguardTeam/Scriptlets/compare/v2.2.16...HEAD
 [#513]: https://github.com/AdguardTeam/Scriptlets/issues/513
+[#545]: https://github.com/AdguardTeam/Scriptlets/issues/545
 
 ## [v2.2.16] - 2026-02-19
 

src/scriptlets/prevent-fetch.js

@@ -168,6 +168,22 @@ export function preventFetch(source, propsToMatch, responseBody = 'emptyObj', re
         return;
     }
 
+    const EXTENSION_SCHEMES = [
+        'chrome-extension:',
+        'moz-extension:',
+        'ms-browser-extension:',
+        'safari-web-extension:',
+    ];
+
+    /**
+     * Check whether a URL uses a known browser extension scheme.
+     * https://github.com/AdguardTeam/Scriptlets/issues/545
+     *
+     * @param {string} url - URL string to check.
+     * @returns {boolean} `true` if the URL starts with a known extension scheme.
+     */
+    const isExtensionScheme = (url) => EXTENSION_SCHEMES.some((scheme) => url.startsWith(scheme));
+
     /**
      * Get the response type based on the given request object.
      *
@@ -210,7 +226,7 @@ export function preventFetch(source, propsToMatch, responseBody = 'emptyObj', re
                 // In the case of apps, the blocked request has status 500
                 // and no error is thrown, so it's necessary to check response.ok
                 // https://github.com/AdguardTeam/Scriptlets/issues/334
-                if (!origResponse.ok) {
+                if (!origResponse.ok || isExtensionScheme(origResponse.url)) {
                     return noopPromiseResolve(strResponseBody, fetchData.url, finalResponseType);
                 }
                 return modifyResponse(

tests/scriptlets/prevent-fetch.test.js

@@ -551,4 +551,36 @@ if (!isSupported) {
         assert.strictEqual(window.hit, 'FIRED', 'hit function fired');
         done();
     });
+
+    test('extension-redirected response should preserve content-length and original URL', async (assert) => {
+        const REQUESTED_URL = `${FETCH_OBJECTS_PATH}/pagead/js/adsbygoogle.js`;
+        const EXTENSION_URL = 'chrome-extension://foo/pagead/js/adsbygoogle.js';
+        const RESPONSE_BODY = 'Response body test';
+
+        // Mock native fetch to simulate browser extension redirect
+        window.fetch = () => {
+            const resp = new Response(RESPONSE_BODY, {
+                status: 200,
+                statusText: 'OK',
+                headers: {
+                    // Extension response is missing Content-Length header
+                },
+            });
+            // Simulate extension-provided URL
+            Object.defineProperty(resp, 'url', { value: EXTENSION_URL });
+            return Promise.resolve(resp);
+        };
+
+        runScriptlet(name, ['adsbygoogle']);
+        const done = assert.async();
+
+        const response = await fetch(REQUESTED_URL);
+
+        const contentLengthHeader = response.headers.get('Content-Length');
+
+        assert.ok(contentLengthHeader !== null, 'Content-Length header is present');
+        assert.strictEqual(response.url, REQUESTED_URL, 'Response URL is the original requested URL');
+        assert.strictEqual(window.hit, 'FIRED', 'hit function fired');
+        done();
+    });
 }