CVE-2020-6821

What it actually does

WebGL copyTexSubImage method returns uninitialized memory contents instead of zero when reading partially or fully outside the source resource, enabling disclosure of sensitive data from browser process memory.

Why it matters to the business

Attackers may obtain private user data such as credentials, browsing history, or other process memory contents by tricking users into visiting malicious websites, increasing risk of further compromise and data loss.

How an attack unfolds

An attacker hosts a malicious webpage containing crafted WebGL JavaScript that invokes copyTexSubImage on out-of-bounds regions; the browser returns uninitialized memory values that the script can exfiltrate to the attacker server.

How to fix it

Upgrade affected Mozilla products to Firefox 75 or later, Firefox ESR 68.7 or later, and Thunderbird 68.7 or later; enable automatic updates where possible and audit enterprise browser deployments.

How to confirm the fix worked

Verify installed browser versions meet or exceed the fixed releases; test WebGL copyTexSubImage behavior with known-good test cases that confirm zeroed output on out-of-bounds reads.

Typical attack path

1. User visits attacker-controlled webpage
2. Page executes JavaScript that sets up WebGL context and calls copyTexSubImage with out-of-bounds parameters
3. Browser returns uninitialized memory instead of zeros
4. Script reads and transmits leaked data back to attacker

How to detect it

Review browser process memory dumps or crash logs for unexpected WebGL texture copy operations; inspect network logs for exfiltration of large binary blobs immediately following WebGL API calls from untrusted origins.

What your security team should do

• Prioritize browser updates across all endpoints within the next 30 days due to public disclosure
• Block or sandbox untrusted web content via enterprise proxy or browser policies until patches are applied
• Monitor for any reported WebGL-related crashes or anomalous memory access in endpoint telemetry