Threats / Mozilla / CVE-2020-6820
CVE-2020-6820
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-06-22
Mozilla Firefox and Thunderbird vulnerability
Mozilla Firefox and Thunderbird contain a race condition in ReadableStream handling that creates a use-after-free vulnerability, enabling arbitrary code execution.
Verdict
Today item — known-exploited.
A race condition vulnerability in ReadableStream processing allows attackers to trigger use-after-free conditions, potentially achieving code execution. Active exploitation in the wild confirms practical weaponization of this defect.
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
2 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03).
Probability (EPSS)
EPSS 0.06305 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
Affected: Mozilla, Firefox and Thunderbird. Confirm exact fixed builds in the vendor advisory.
Weakness (CWE)
Mapped to CWE-362 Race Condition.CWE assignment from the public NVD record; the weakness class drives how the flaw is exploited.
WeaknessCWE-362 · Race Condition
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious webpage or document that triggers concurrent ReadableStream operations to create a race condition.
Business
Users visiting untrusted content face immediate risk of browser compromise and system takeover.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I exploit the use-after-free condition to read or write memory, bypassing security protections.
Business
Confidential data stored in browser memory becomes accessible; sandbox escapes enable host system compromise.
3
Lateral reach — past segmentation narrative 3
Attacker
I execute arbitrary code within the browser process or escalate privileges on the victim system.
Business
Attackers gain persistent access to user devices, enabling data theft, malware installation, and lateral movement.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05