Cyber Resilience

CVE-2020-6819

Memory Safety in Mozilla Firefox ≤ 68.6.1

CISA KEVActive ExploitationEUVD ExploitedPublic PoCMemory SafetyRace Condition
Published
24 April 2020
Modified
04 November 2025
KEV Added
03 November 2021
Patch / advisory
View advisory
CVSS Score v3.1 8.1
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS Score 0.030 86th percentile
Risk Priority 86 floored blend · peak EPSS

Summary

CVE-2020-6819 is a high-severity Race Condition (CWE-362) vulnerability in Mozilla Firefox. Its CVSS base score is 8.1 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Client Execution (T1203); ranked in the top 14% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog; a public proof-of-concept is referenced.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2020-6819 is a use-after-free vulnerability resulting from a race condition during execution of the nsDocShell destructor. It affects Mozilla Firefox versions prior to 74.0.1, Firefox ESR versions prior to 68.6.1, and Thunderbird versions prior to 68.7.0, and is tracked under CWEs 362 and 416.

An unauthenticated remote attacker can trigger the flaw over the network with no user interaction required, although exploitation complexity is rated high. Successful abuse grants the attacker control over memory contents, enabling impacts to confidentiality, integrity, and availability.

Mozilla security advisories MFSA2020-11 and MFSA2020-14, along with corresponding Ubuntu updates, direct users to apply the fixed releases (Firefox 74.0.1, ESR 68.6.1, Thunderbird 68.7.0) as the primary mitigation.

Targeted attacks abusing this issue have been observed in the wild.

OWASP Top 10 for Web (2025)

A06:2025 Insecure Design

EU & UK References

Vulnerability Data

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 <…

more

68.6.1.

CWE(s)
CWE-362CWE-416
KEV Date Added
03 November 2021

Related Threats

MITRE ATT&CK Enterprise Techniques

T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2022-26486Same product: Mozilla Firefoxboth on KEV
CVE-2022-26485Same product: Mozilla Firefoxboth on KEV
CVE-2026-2765Same product: Mozilla Firefox
CVE-2026-2767Same product: Mozilla Firefox
CVE-2026-2770Same product: Mozilla Firefox
CVE-2024-5702Same product: Mozilla Firefox
CVE-2024-4770Same product: Mozilla Firefox
CVE-2026-0884Same product: Mozilla Firefox
CVE-2025-1930Same product: Mozilla Firefox
CVE-2026-2799Same product: Mozilla Firefox

Affected Assets

mozilla
firefox
≤ 68.6.1 · ≤ 74.0.1
mozilla
thunderbird
≤ 68.7.0

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)
  • 3 hardening rules · 3 OS baselines
Validate
Prove the fix (OWASP ASVS)
  • V10.4.2
  • V10.4.5
  • V15.1.3
  • V15.4.1

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

AU-8 Time Stamps
addresses: CWE-362

Accurate timestamps from internal clocks enable detection of race conditions by providing reliable event ordering in audit logs.

PL-6 Security-related Activity Planning
addresses: CWE-362

Coordination of concurrent security activities reduces the probability that shared resources will be accessed simultaneously without proper synchronization.

SI-16 Memory Protection
addresses: CWE-416

Use-after-free exploits that achieve arbitrary code execution are blocked or significantly hardened by non-executable pages and ASLR.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper synchronization primitives and concurrency testing that prevent race conditions.

ID.RA-01 partial match
prevents

Vulnerability identification processes can discover use-after-free issues via scanning or analysis but do not prevent their introduction.

PR.PS-02 partial match
prevents

Routine patching removes known use-after-free instances after they have been introduced in released software.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect race conditions, but does not prevent them at design or coding time.

A.8.25 Secure development life cycle partial match
prevents

Secure SDLC mandates concurrency controls and synchronization primitives that directly prevent race conditions.

A.8.26 Application security requirements partial match
prevents

Application security requirements can specify thread-safety and locking rules, but do not prescribe implementation details.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles require proper synchronization and resource isolation, addressing the root cause of CWE-362.

A.8.28 Secure coding partial match
prevents

Secure coding standards explicitly forbid unsafe concurrent access patterns and mandate atomic operations or locks.

A.8.32 Change management partial match
prevents

Change-management processes help ensure memory-safety fixes are deployed consistently.

Hardening callouts derived

Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).

Oracle Linux 8 (1 rule)
  • V-248592 OL 8 must clear memory when it is freed to prevent use-after-free attacks. prevents CWE-416
RHEL 8 (1 rule)
  • V-230279 RHEL 8 must clear memory when it is freed to prevent use-after-free attacks. prevents CWE-416
RHEL 9 (1 rule)
  • V-257794 RHEL 9 must clear memory when it is freed to prevent use-after-free attacks. prevents CWE-416

References