CVE-2020-6820 — Mozilla
HIGHCVE-2020-6820 is a high-severity (CVSS 8.1) CWE-362 vulnerability. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-11-03. Federal agencies were required to remediate by 2022-05-03.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2020-6820.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2020-6820 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 395,249 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Description
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.
Affected Products
firefoxmozillathunderbirdmozillaResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
VulnerabilityDetect
Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).
Remediation status
No fixed release is recorded for CVE-2020-6820 yet. Track the upstream advisory, and apply the exposure-reduction steps below in the meantime.
Mitigate without a patch
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 6 | firefox-0:68.6.1-1.el6_10 | RHSA-2020:1339 |
| Red Hat Enterprise Linux 6 | thunderbird-0:68.7.0-1.el6_10 | RHSA-2020:1488 |
| Red Hat Enterprise Linux 7 | firefox-0:68.6.1-1.el7_8 | RHSA-2020:1338 |
| Red Hat Enterprise Linux 7 | thunderbird-0:68.7.0-1.el7_8 | RHSA-2020:1489 |
| Red Hat Enterprise Linux 8 | firefox-0:68.6.1-1.el8_1 | RHSA-2020:1341 |
| Red Hat Enterprise Linux 8 | thunderbird-0:68.7.0-1.el8_1 | RHSA-2020:1495 |
| Red Hat Enterprise Linux 8.0 Update Services for SAP Solutions | firefox-0:68.6.1-1.el8_0 | RHSA-2020:1340 |
| Red Hat Enterprise Linux 8.0 Update Services for SAP Solutions | thunderbird-0:68.7.0-1.el8_0 | RHSA-2020:1496 |
Frequently Asked Questions
Is CVE-2020-6820 in your dependencies?
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