CVE-2021-38491

What it actually does

Mixed-content checks were unable to analyze opaque origins which led to some mixed content being loaded, bypassing intended security restrictions on HTTP content within HTTPS pages.

Why it matters to the business

Attackers could tamper with page content or bypass content integrity protections in affected browsers, potentially leading to altered user experiences or phishing risks on web applications.

How an attack unfolds

An attacker hosts a secure HTTPS page that references HTTP subresources via opaque origins; the browser's flawed mixed-content validation fails to block the insecure loads, allowing the mixed content to execute.

How to fix it

Upgrade Mozilla Firefox to version 92 or later to restore proper mixed-content origin analysis.

How to confirm the fix worked

Confirm installed Firefox version is 92 or higher and verify that mixed-content blocking functions correctly for opaque origins in test scenarios.

Typical attack path

1. Craft HTTPS webpage embedding HTTP resources using opaque origins
2. Victim browser loads the page without triggering mixed-content blocks
3. Insecure content executes, enabling content modification or further attacks

How to detect it

Inspect browser console logs and network captures for unblocked HTTP subresource loads on HTTPS pages; look for absence of expected mixed-content warnings on pages known to include such resources.

What your security team should do

• Prioritize updating all Firefox deployments to 92 or newer
• Enforce HTTPS-only policies and content security headers as defense-in-depth
• Monitor browser update compliance across the environment