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Full vulnerability report · 2020
CVE-2020-6819High confidence

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Mozilla · Firefox and Thunderbird

8.1HighCVSS 3.1
Recommended action
Protect now

CISA confirms exploitation in the wild and lists 2022-05-03 as the remediation due date.

Fix not verified
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Select the national-authority views to include. The exact source language is shown on each matched advisory. Your choice is remembered on this device and encoded in the shareable URL.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2020-27963

CISA KEV mirrored by ENISA

Under certain conditions, when running the nsDocShell destructor, 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.

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 3 Nov 2021. Evidence sources: cisa_kev.
ENISA score
8.1 · CVSS 3.1
Advisory evidence
17 linked advisory records
Explicit mitigation evidence

Only statements that explicitly mention a fix, patch, update, workaround or mitigation are shown here.

  • csaf_opensuse · openSUSE-SU-2020:0544-1Security update for MozillaThunderbird
  • csaf_suse · SUSE-SU-2020:0929-1Security update for MozillaFirefox
  • csaf_redhat · RHSA-2020:1339Red Hat Security Advisory: firefox security update
  • csaf_opensuse · openSUSE-SU-2020:0520-1Security update for MozillaThunderbird
  • csaf_opensuse · openSUSE-SU-2020:0461-1Security update for MozillaFirefox
  • csaf_suse · SUSE-SU-2020:14337-1Security update for MozillaFirefox
  • csaf_redhat · RHSA-2020:1489Red Hat Security Advisory: thunderbird security update
  • csaf_suse · SUSE-SU-2020:0928-1Security update for MozillaFirefox
  • csaf_suse · SUSE-SU-2020:1027-1Security update for MozillaThunderbird
  • csaf_redhat · RHSA-2020:1488Red Hat Security Advisory: thunderbird security update
  • csaf_redhat · RHSA-2020:1496Red Hat Security Advisory: thunderbird security update
  • csaf_redhat · RHSA-2020:1341Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1340Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1338Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1495Red Hat Security Advisory: thunderbird security update
Recommended actionProtect now

CISA confirms exploitation in the wild and lists 2022-05-03 as the remediation due date.

Fix not verified
01

What, why and how

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

What

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

Why

The program can continue using memory after it has been released, producing unsafe and attacker-influenceable behaviour.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

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

Why

The program can continue using memory after it has been released, producing unsafe and attacker-influenceable behaviour.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Reference recorded

A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.

Likely attack path
a network path → Use After Free → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
High: exploitation depends on specific conditions
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-416 ↗

CWE-416: Use After Free. The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACHighAttack complexity: Successful exploitation depends on specific conditions outside the attacker's direct control.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedCWE-416CISA KEVPublic exploit reference
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2020-27963Known-exploited evidence recorded

Under certain conditions, when running the nsDocShell destructor, 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.

Official EUVD record
CERT-FR · French · CERTFR-2020-AVI-204Multiples vulnérabilités dans Mozilla Thunderbird

De multiples vulnérabilités ont été découvertes dans Mozilla Thunderbird. Elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance et une atteinte à la confidentialité des données.

Official advisory
CERT-FR · French · CERTFR-2020-AVI-187Multiples vulnérabilités dans Mozilla Firefox

De multiples vulnérabilités ont été découvertes dans Mozilla Firefox. Elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance.

Official advisory
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix not verified
Affected
unspecified < 68.7.0; unspecified < 74.0.1; unspecified < 68.6.1
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
No verified patch reference is present in the current structured sources. Check the vendor advisory before making a change.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 24 Apr 2020 · Last source change 21 Oct 2025, 23:35 UTC · CWE-416 · Use After Free

CVE recordCVE.org · 5.1
CVSS sourceNIST NVD
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2020-27963
Product sourceCNA
Remediation sourceCVE/CNA references
CWE sourceCISA ADP
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates →

No material field changes have been recorded since change tracking began. Routine source refreshes and cosmetic edits are intentionally excluded.

Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
ENISA EUVD · EUVD-2020-27963↗CISA KEV record↗CVE authority or vendor reference↗Vendor advisory↗Public exploit reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2020-6819 · cve.blacktree.nl