CVE-2020-6819
CVE-2020-6819 is a security flaw in Mozilla Firefox and Thunderbird, Mozilla’s web browser. Attackers can corrupt memory in Mozilla Firefox and Thunderbird, which they use to take it over. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021.[1]
Mozilla Firefox and Thunderbird is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Fixed in version 68.7.0, 74.0.1 and 68.6.1.
Description
CISA calls it an use-after-free in Mozilla Firefox and Thunderbird. In its words: “Mozilla Firefox and Thunderbird contain a race condition vulnerability when running the nsDocShell destructor under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.”[1]
The CVE record adds: “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 < 68.6.1.”[2]
The underlying weakness is race condition (CWE-362), use after free (CWE-416), in MITRE’s list of common software weaknesses.
It scores 8.1 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Thunderbird | 68.7.0 |
| Firefox | 74.0.1 |
| Firefox ESR | 68.6.1 |
Every device with Mozilla Firefox and Thunderbird needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 558 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 May 2022 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 3%. That’s in the top 13% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Mozilla has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Update Mozilla Firefox and Thunderbird now.Install the latest update from Mozilla on every device that has it, and restart when asked. Then turn on automatic updates, so the next fix arrives on its own.
- Check you’re on 68.7.0 or later.Fixed in version 68.7.0, 74.0.1 and 68.6.1. Anything older is what attackers are going after.
- If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.1.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 3%.[4] |
See also
Other Mozilla Firefox and Thunderbird flaws
- CVE-2019-11708: sandbox escape ·
- CVE-2019-11707: type confusion ·
- CVE-2013-1690: denial-of-service ·
- CVE-2020-6820: use-after-free ·
- CVE-2019-17026: type confusion ·
Other Mozilla flaws
- CVE-2010-3765, Mozilla products: remote code execution ·
- CVE-2024-9680, Mozilla Firefox: use-after-free ·
- CVE-2016-9079, Mozilla Firefox, Firefox ESR, and Thunderbird: use-after-free ·
- CVE-2015-4495, Mozilla Firefox: security feature bypass ·
- CVE-2022-26486, Mozilla Firefox: use-after-free ·
Other race condition flaws
- CVE-2025-39964, Linux Kernel: race condition ·
- CVE-2025-62215, Microsoft Windows: race condition ·
- CVE-2023-36884, Microsoft Windows: search remote code execution ·
- CVE-2021-25395, Samsung Mobile Devices: race condition ·
- CVE-2014-0196, Linux Kernel: race condition ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Mozilla flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Mozilla Firefox And Thunderbird Use-After-Free Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2020-6819 Detail”. National Vulnerability Database, published 24 Apr 2020.
- ^ Mozilla. Advisories and fixes: mozilla.org, mozilla.org.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.030, percentile 87.5, 31 Oct 2026.
- CVE Program. “CVE-2020-6819”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.