# CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

- Exploited (CISA KEV): yes, added 2021-11-03
- Published: 2020-04-24
- CVSS: 8.1 HIGH (v3.1)
- EPSS: 7.1% (94th percentile)
- CWE: CWE-362

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

## Edge WAF coverage (claimed by vendors, not measured)

| Vendor | Status | First rule | Days after publication |
|---|---|---|---|
| Cloudflare | Before feed history | — | — |
| AWS | No public claim | — | — |
| Microsoft Azure | No public claim | — | — |
| Google Cloud | Before feed history | — | — |
| Fastly | Before feed history | — | — |
| F5 | Before feed history | — | — |
| Akamai | No public source | — | — |
| OWASP CRS | Before feed history | — | — |

"No public claim" means no vendor statement names this CVE; "Before feed history" means the CVE is older than the vendor feed. In both cases generic rules may still block it.

HTML: https://cve.vigilbase.com/cve/CVE-2020-6820 · JSON: https://cve.vigilbase.com/api/v1/cves/CVE-2020-6820