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Full vulnerability report · 2020
CVE-2020-6820High 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
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 3 Nov 2021

Evidence used

  • CISA confirms exploitation in the wild.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 7.06% for the current model date.

Compensating controls

  • Restrict the affected network interface to trusted sources where business-safe.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Mozilla Firefox and Thunderbird and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Distribution package intelligence

Release-specific package status

Alpine, Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

21 package states
Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Alpine v3.23v3.23 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.23v3.23 · communityfirefox-esrVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.6.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.23v3.23 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.23v3.23 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.7.0-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communityfirefox-esrVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.6.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.7.0-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.21v3.21 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 19 Aug 2026
Alpine v3.21v3.21 · communityfirefox-esrVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.6.1-r0Alpine Security Database ↗Source updated 19 Aug 2026
Alpine v3.21v3.21 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.74.0.1-r0Alpine Security Database ↗Source updated 19 Aug 2026
Alpine v3.21v3.21 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.68.7.0-r0Alpine Security Database ↗Source updated 19 Aug 2026
Debian trixietrixie · sourcefirefox-esrVendor fix publishedDebian records a fixed source-package version for this release.68.6.1esr-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian trixietrixie · sourcethunderbirdVendor fix publishedDebian records a fixed source-package version for this release.1:68.7.0-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian bookwormbookworm · sourcefirefox-esrVendor fix publishedDebian records a fixed source-package version for this release.68.6.1esr-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian bookwormbookworm · sourcethunderbirdVendor fix publishedDebian records a fixed source-package version for this release.1:68.7.0-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian forkyforky · sourcefirefox-esrVendor fix publishedDebian records a fixed source-package version for this release.68.6.1esr-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian forkyforky · sourcethunderbirdVendor fix publishedDebian records a fixed source-package version for this release.1:68.7.0-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcefirefoxVendor fix publishedDebian records a fixed source-package version for this release.74.0.1-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcefirefox-esrVendor fix publishedDebian records a fixed source-package version for this release.68.6.1esr-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcethunderbirdVendor fix publishedDebian records a fixed source-package version for this release.1:68.7.0-1Debian Security Tracker ↗Source updated 6 Oct 2026
Optional official sources

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

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

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-2020-27964

CISA KEV mirrored by ENISA

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.

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_redhat · RHSA-2020:1495Red Hat Security Advisory: thunderbird security update
  • csaf_redhat · RHSA-2020:1489Red Hat Security Advisory: thunderbird security update
  • csaf_redhat · RHSA-2020:1488Red Hat Security Advisory: thunderbird security update
  • csaf_opensuse · openSUSE-SU-2020:0520-1Security update for MozillaThunderbird
  • csaf_redhat · RHSA-2020:1341Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1338Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1496Red Hat Security Advisory: thunderbird security update
  • csaf_suse · SUSE-SU-2020:1027-1Security update for MozillaThunderbird
  • csaf_opensuse · openSUSE-SU-2020:0544-1Security update for MozillaThunderbird
  • csaf_suse · SUSE-SU-2020:0928-1Security update for MozillaFirefox
  • csaf_suse · SUSE-SU-2020:0929-1Security update for MozillaFirefox
  • csaf_opensuse · openSUSE-SU-2020:0461-1Security update for MozillaFirefox
  • csaf_suse · SUSE-SU-2020:14337-1Security update for MozillaFirefox
  • csaf_redhat · RHSA-2020:1340Red Hat Security Advisory: firefox security update
  • csaf_redhat · RHSA-2020:1339Red Hat Security Advisory: firefox 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 handling a ReadableStream 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 handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

Why

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

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 handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

Why

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

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.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') → 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-362 ↗

CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'). The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

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-362CISA KEV
A

Official authority intelligence

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

ENISA EUVD · EUVD-2020-27964Known-exploited evidence recorded

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.

Official EUVD record ↗
Cyber Security Agency of Singapore · English · CSA-SB-20200429Security Bulletin 29 Apr 2020

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 29 Apr 2020, published on 29 April 2020. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
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 ↗
JVN iPedia · Japanese · JVNDB-2020-004939Mozilla Firefox および Thunderbird における二重解放に関する脆弱性

Mozilla Firefox および Thunderbird には、二重解放に関する脆弱性が存在します。

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
  • Thunderbird: unspecified < 68.7.0
  • Firefox: unspecified < 74.0.1
  • Firefox ESR: 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-362 · Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

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-2020-27964
Product sourceCISA KEV
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 ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    unspecified < 68.7.0; unspecified < 74.0.1; unspecified < 68.6.1 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    Thunderbird: unspecified < 68.7.0; Firefox: unspecified < 74.0.1; Firefox ESR: unspecified < 68.6.1 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
ENISA EUVD · EUVD-2020-27964↗CISA KEV record↗CVE authority or vendor reference↗Vendor advisory↗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-2020-6820 · cve.blacktree.nl