Why Did Our Banner Turn Gray? A Real‑World Debugging Tale of AdGuard Blocking Chrome

Problem Description

A user reported that after the Double‑11 promotional launch, the rotating banner on the web forum displayed only a gray placeholder. The issue was observed on Chrome 86.0.4240.111 running on macOS 10.15.7.

Environment and Initial Reproduction

The test engineer recreated the exact environment (Chrome 86, macOS 10.15.7) and followed the same navigation steps. No error could be reproduced; the banner loaded normally.

Additional Checks

Network conditions were throttled to simulate a weak connection – the banner still rendered.

CDN endpoints were verified with curl and browser devtools; responses were correct.

Browser cache was cleared, extensions were disabled, and proxy settings were removed – the problem persisted for the user.

Video Analysis and Discovery

The user supplied a screen‑recording. A close inspection revealed a small overlay with the number “2” in the Chrome toolbar, indicating the presence of the AdGuard extension from the Chrome Web Store.

Reproduction with AdGuard

Installing AdGuard in the test browser reproduced the exact gray‑banner behavior. The extension’s log (shown below) listed the banner image URLs as blocked resources.

Root Cause

AdGuard’s filter rules matched the banner image URLs (e.g., https://cdn.example.com/banner/*.jpg) and blocked them, causing the browser to render a gray placeholder. Because the extension operates before the page’s own scripts, the site could not recover the images.

Mitigation

Two complementary actions were taken:

Adjust the site’s Content‑Security‑Policy (CSP) to explicitly allow the banner domain, reducing the chance that generic ad‑blocking rules interfere.

Content-Security-Policy: img-src 'self' https://cdn.example.com; script-src 'self' 'unsafe-inline';

Provide user guidance for AdGuard users: add the site’s domain to the AdGuard whitelist or disable the specific filter that blocks the banner images.

Verification

After deploying the updated CSP header and informing users, the banner rendered correctly even with AdGuard enabled. Subsequent monitoring confirmed that no further gray‑banner reports were received.