Threats / Mozilla / CVE-2020-6819
CVE-2020-6819
· 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 the nsDocShell destructor that creates a use-after-free vulnerability, enabling arbitrary code execution.
Verdict
Today item — known-exploited.
A race condition vulnerability in Firefox and Thunderbird allows attackers to trigger use-after-free conditions during object destruction, potentially achieving code execution. The vulnerability has been exploited in the wild.
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 0.02978 — 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-416 Use After Free — weakness family: Memory safety.CWE assignment from the public NVD record; the weakness class drives how the flaw is exploited.
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 email that triggers concurrent access to the nsDocShell destructor under specific timing conditions.
Business
Users visiting compromised websites or opening malicious email attachments face arbitrary code execution risk, compromising system integrity and data confidentiality.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I exploit the race condition to access freed memory regions, allowing me to read sensitive data or overwrite critical structures.
Business
Attackers gain unauthorized access to user credentials, browsing history, and personal information stored in browser memory.
3
Lateral reach — past segmentation narrative 3
Attacker
I leverage the use-after-free to execute arbitrary code with the privileges of the browser process.
Business
System compromise enables lateral movement, malware installation, and persistent unauthorized access to affected endpoints.
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