CVE-2020-6820
Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
Description
CVE-2020-6820 is a high-severity use-after-free vulnerability affecting Mozilla Firefox and Thunderbird. A race condition occurs when handling a ReadableStream, which can lead to a use-after-free condition that allows attackers to potentially execute arbitrary code or cause other unspecified impacts. Mozilla has confirmed awareness of targeted attacks in the wild exploiting this flaw. CISA has added CVE-2020-6820 to its Known Exploited Vulnerabilities catalog, and with an EPSS score of 5.0% (89.6th percentile), this vulnerability poses a significant exploitation risk to organizations running unpatched versions of Firefox or Thunderbird.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected 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=1626728(Issue Tracking, Permissions Required)
- 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-2020-6820(US Government Resource)
Weakness Type
CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization ('Race Condition')
CWE-362 describes a race condition that occurs when multiple threads or processes access a shared resource without proper synchronization. In the case of CVE-2020-6820, a race condition in Mozilla Firefox and Thunderbird during ReadableStream handling causes memory to be freed prematurely while still in use, resulting in a use-after-free condition.
Learn more: CWE-362 — Concurrent Execution Using Shared Resource with Improper Synchronization ('Race Condition')
Impact Analysis
CVE-2020-6820 carries a CVSS 3.1 score of 8.1 (HIGH), reflecting serious security implications. The vulnerability is exploitable over the network without requiring authentication or user interaction, though the attack complexity is high due to the race condition that must be precisely triggered. Confidentiality (High): A successful exploit can lead to full disclosure of sensitive data processed by the browser. Integrity (High): Attackers may gain the ability to modify data or inject malicious content within the browser context. Availability (High): Exploitation can result in browser crashes or complete denial of service. The EPSS score of 5.0% places this vulnerability in the 89.6th percentile, indicating a notable probability of exploitation compared to other known vulnerabilities.
Exploit Maturity
CVE-2020-6820 is confirmed as actively exploited in the wild, as acknowledged by Mozilla in its security advisory and further corroborated by its inclusion in the CISA Known Exploited Vulnerabilities catalog. The EPSS score of 5.0% (89.6th percentile) underscores a meaningful likelihood of exploitation activity. Although the bug tracker entry at Bugzilla is restricted, the vulnerability was disclosed alongside CVE-2020-6819, and both flaws were reportedly exploited together in targeted attack campaigns. Federal agencies were required to remediate this vulnerability by 2022-05-03 per CISA's binding operational directive.
Remediation
- Apply vendor patches immediately as required by CISA KEV: Apply updates per vendor instructions. Update Mozilla 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 deployed instances of Firefox and Thunderbird across the organization have been updated by auditing installed browser and email client versions against the patched releases.
- If immediate patching is not feasible, consider restricting access to untrusted web content through network-level filtering, web proxy policies, or disabling JavaScript execution on untrusted sites to reduce the attack surface.
- Monitor browser process behavior for anomalous activity such as unexpected crashes, unusual memory consumption patterns, or suspicious child process spawning that may indicate exploitation attempts.
- Conduct threat hunting and review security logs to determine whether this vulnerability was exploited prior to patching, paying particular attention to any indicators associated with the targeted attacks reported by Mozilla.
Technical Details
CVE-2020-6820 is a use-after-free vulnerability triggered by a race condition (CWE-362) in Mozilla Firefox and Thunderbird when handling a ReadableStream object. The flaw arises when concurrent operations on a ReadableStream cause a destructor to execute while the stream's memory is still being referenced by another thread, leading to a dangling pointer that can be dereferenced by the attacker. The CVSS vector (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H) indicates that while the vulnerability is remotely exploitable without authentication, the attacker must win a race condition (high attack complexity) to trigger the use-after-free. Once triggered, the attacker gains full control over confidentiality, integrity, and availability of the affected process, potentially enabling arbitrary code execution within the browser's sandbox context.
Frequently Asked Questions
Is CVE-2020-6820 being actively exploited?
Yes. Mozilla has confirmed targeted attacks in the wild exploiting CVE-2020-6820, and CISA has listed it in the Known Exploited Vulnerabilities catalog. The EPSS score of 5.0% (89.6th percentile) further supports that exploitation is actively occurring.
What products are affected by CVE-2020-6820?
CVE-2020-6820 affects Mozilla Firefox versions prior to 74.0.1, Firefox ESR versions prior to 68.6.1, and Mozilla Thunderbird versions prior to 68.7.0. All platforms running these browser and email client versions are potentially vulnerable.
How do I fix CVE-2020-6820?
Update Mozilla Firefox to version 74.0.1 or later, Firefox ESR to 68.6.1 or later, and Thunderbird to 68.7.0 or later. If patching is not immediately possible, restrict exposure to untrusted web content and monitor for unusual browser behavior.
How severe is CVE-2020-6820?
CVE-2020-6820 has a CVSS 3.1 score of 8.1 (HIGH). While the attack complexity is high due to the race condition requirement, the full impact on confidentiality, integrity, and availability makes this a serious vulnerability that requires prompt remediation.
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