CVE-2020-6819
Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
Description
CVE-2020-6819 is a high-severity use-after-free vulnerability in Mozilla Firefox and Thunderbird caused by a race condition during the execution of the nsDocShell destructor. Under certain conditions, a race condition in the browser's document shell component leads to a use-after-free condition that can be exploited for arbitrary code execution. Mozilla has confirmed awareness of targeted attacks in the wild abusing this flaw. CISA has added CVE-2020-6819 to its Known Exploited Vulnerabilities (KEV) catalog. With an EPSS score of 0.33% (55.5th percentile), the exploitation probability is moderate but the confirmed in-the-wild targeting makes immediate patching critical.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorCWEs
Affected Products
| Vendor | Product | Version |
|---|---|---|
| mozilla | firefox | < 68.6.1; < 74.0.1 |
| mozilla | thunderbird | < 68.7.0 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1620818(Exploit, Issue Tracking)
- https://usn.ubuntu.com/4335-1/(Third Party Advisory)
- https://www.mozilla.org/security/advisories/mfsa2020-11/(Vendor Advisory)
- https://www.mozilla.org/security/advisories/mfsa2020-14/(Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6819(US Government Resource)
Weakness Type
CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization ('Race Condition')
A race condition occurs when two or more threads access shared data concurrently and the outcome depends on the timing of their execution. In CVE-2020-6819, the nsDocShell destructor in Firefox and Thunderbird is subject to a race condition where concurrent operations on shared browser state lead to an exploitable memory corruption condition.
Learn more: CWE-362 — Concurrent Execution Using Shared Resource with Improper Synchronization
CWE-416: Use After Free
Use After Free occurs when software references memory after it has been freed, which can lead to arbitrary code execution, data corruption, or crashes. In CVE-2020-6819, the race condition in the nsDocShell destructor causes a memory region to be freed while still being referenced, creating a use-after-free condition that attackers have exploited in targeted attacks.
Learn more: CWE-416 — Use After Free
Impact Analysis
CVE-2020-6819 carries a CVSS 3.1 score of 8.1 (HIGH), indicating a critical threat to users of Mozilla Firefox and Thunderbird. The vulnerability is remotely exploitable over the network without requiring authentication or user interaction, though the attack complexity is high due to the race condition timing requirements. Successful exploitation results in full compromise of confidentiality, integrity, and availability, as the use-after-free condition can be leveraged for arbitrary code execution within the browser process. Mozilla has explicitly stated awareness of targeted attacks abusing this flaw, confirming real-world exploitation against specific targets. Despite the relatively modest EPSS score of 0.33% (55.5th percentile), the confirmed targeted exploitation in the wild makes this vulnerability a high priority for remediation, particularly for organizations and individuals who may be targets of advanced threat actors.
Exploit Maturity
CISA has confirmed active exploitation of CVE-2020-6819 by including it in the Known Exploited Vulnerabilities catalog, and Mozilla's own advisory states that the vendor is "aware of targeted attacks in the wild abusing this flaw." The vulnerability is tracked in Mozilla Bugzilla where exploit details have been documented. The EPSS score of 0.33% (55.5th percentile) suggests moderate overall exploitation probability, but the confirmed targeted attacks indicate that sophisticated threat actors possess working exploits. CVE-2020-6819 is closely related to CVE-2020-6820, another use-after-free vulnerability in Firefox and Thunderbird disclosed at the same time, and the two may be chained together for more reliable exploitation.
Remediation
- Update Mozilla Firefox and Thunderbird immediately as required by CISA KEV guidance. Upgrade Firefox to version 74.0.1 or later, Firefox ESR to version 68.6.1 or later, and Thunderbird to version 68.7.0 or later.
- Verify that all instances of affected products have been updated across the organization. Use software inventory tools to identify systems running vulnerable versions of Firefox (below 74.0.1 or ESR below 68.6.1) and Thunderbird (below 68.7.0).
- As an interim mitigation, consider restricting browser functionality through enterprise policies that limit JavaScript execution or enforce content security policies on high-risk web pages. Implement browser isolation solutions for users who may be targets of advanced persistent threats.
- Monitor endpoint logs for suspicious browser process behavior, including unexpected child processes spawned by Firefox or Thunderbird, unusual memory access patterns, and signs of exploitation such as crash reports or abnormal resource consumption.
- Review the related vulnerability CVE-2020-6820 and ensure both patches are applied simultaneously, as these vulnerabilities were disclosed together and may be chained in attack scenarios.
Technical Details
CVE-2020-6819 is rooted in a race condition within the nsDocShell destructor in Mozilla Firefox and Thunderbird. The nsDocShell is a core browser component responsible for managing document loading and navigation within browser frames. Under certain conditions, when the nsDocShell destructor executes concurrently with other operations accessing the same shared state, a race condition arises that causes memory to be freed while still in use, resulting in a use-after-free vulnerability. The CVSS vector (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H) confirms that the vulnerability is network-exploitable without authentication or user interaction, but the high attack complexity reflects the challenge of reliably winning the race condition. The combination of CWE-362 (race condition) and CWE-416 (use-after-free) illustrates how the timing flaw directly enables the memory safety violation, with the race condition serving as the root cause that leads to the exploitable use-after-free state.
Frequently Asked Questions
Is CVE-2020-6819 being actively exploited?
Yes. CVE-2020-6819 is listed in the CISA Known Exploited Vulnerabilities catalog, and Mozilla has confirmed awareness of targeted attacks in the wild. While the EPSS score of 0.33% suggests moderate overall exploitation probability, the confirmed targeted attacks demonstrate active exploitation by sophisticated threat actors.
What products are affected by CVE-2020-6819?
CVE-2020-6819 affects Mozilla Firefox versions below 74.0.1, Firefox ESR versions below 68.6.1, and Mozilla Thunderbird versions below 68.7.0. All platforms where these browsers are installed are potentially vulnerable.
How do I fix CVE-2020-6819?
Update Firefox to version 74.0.1 or later, Firefox ESR to 68.6.1 or later, and Thunderbird to 68.7.0 or later. Also apply the patch for the related CVE-2020-6820, as both vulnerabilities were disclosed simultaneously and may be chained in attacks.
How severe is CVE-2020-6819?
CVE-2020-6819 has a CVSS 3.1 score of 8.1 (HIGH). It enables remote code execution through a race condition and use-after-free, and Mozilla has confirmed targeted attacks exploiting this flaw in the wild, making prompt patching essential.
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