Vulnerability

CVE-2020-6820

High· 8.1KEVCVSS8.1EPSS3%

.rules3/5

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

Threat signal

91/100

Severe

Severity

high

CVSS

8.1

EPSS

Active

KEV

CISA KEV — actively exploited

1502d overdue

AffectedMozilla — Firefox and Thunderbird

Added to KEV

2021-11-03

Federal patch due (BOD 22-01)

2022-05-03

Required action (CISA verbatim)

Apply updates per vendor instructions.

Vendor advisory + references

https://nvd.nist.gov/vuln/detail/CVE-2020-6820

CISA description: “Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.”

CVSS

v3.1Open in FIRST calculator →

8.1/ 10HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack VectorNetworkExploitable remotely over the internet — no local access needed.
Attack ComplexityHighRequires conditions outside attacker's control (race, brute-force, configuration
Privileges RequiredNoneUnauthenticated — no credentials needed.
User InteractionNoneFully automated — no victim action needed.
ScopeUnchangedImpact contained to the vulnerable component.
Confidentiality ImpactHighTotal disclosure of sensitive data.
Integrity ImpactHighTotal compromise of integrity — full data modification possible.
Availability ImpactHighTotal loss of availability — full DoS possible.

EPSS

Exploit prediction · 30-day horizonFIRST.org →

3.13%

probability of exploitation in next 30 days

p87

higher than 87% of all CVEs

3-day trend↑ 0pp

Mid-pack — moderate exploitation likelihood.

Vendor VEX

No VEX statements published for CVE-2020-6820. Vendors publish VEX (Vulnerability Exploitability eXchange) to assert per-product whether a CVE is actually exploitable in their distribution.

SSVCAct

Active exploitation with total impact on essential/support systems

exploitation: active
automatable: yes
impact: total
mission: support

Lifecycle

2020-05-20Vendor advisory · amazon-linuxALAS2-2020-1429 — ALAS2-2020-1429 (critical): thunderbird
2021-11-03Added to CISA KEVransomware: Unknown
2021-11-03Published
2022-05-03Federal patch due (BOD 22-01)
2026-06-12Last modified1 source

Entity graph

2 nodes · 1 edges · click to pivot

CVEActorVendor advisoryIOCNews

Vendor advisories

amazon-linuxALAS2-2020-1429ALAS2-2020-1429 (critical): thunderbird

No source-event history recorded yet. New revisions will appear here as ingestors re-fetch this CVE.