CVE-2020-6820
HighExploitedPublic exploitUnder certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.
Published Apr 24, 2020 · Updated Aug 5, 2026 · Also known as DEBIAN-CVE-2020-6820, UBUNTU-CVE-2020-6820
Key Facts
Patch immediately- CVE-2020-6820 is a high-severity vulnerability (CVSS 8.1 v3.1) affecting Firefox (Mozilla), Thunderbird (Mozilla), Firefox ESR (alpine) and 15 more products.
- It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Apr 1, 2020. Public exploit code is available.
- EPSS estimates a 7.1% probability of exploitation in the next 30 days.
- Fixed versions are available, including Firefox 0:68.6.1-1.el6_10, Firefox 0:68.6.1-1.el7_8, Firefox 0:68.6.1-1.el8_0.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
8.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
7.1%
Higher than 94.1% of all scored CVEs
Exploited in the wild
Yes
4 catalog listings
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2020-6820 brief as a PDF
Every affected product and fixed release, exploitation status and a first 24 hours response checklist, emailed within minutes.
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.
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| CISA KEV | Nov 3, 2021 | May 3, 2022 | Not reported |
| Industry KEV feed | Apr 1, 2020 | May 3, 2022 | Not reported |
| CIRCL KEV | Nov 3, 2021 | May 3, 2022 | Not reported |
| ENISA EUVD | Nov 3, 2021 | n/a | Not reported |
Required action: Apply updates per vendor instructions.
Affected Products
185 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| FirefoxMozilla | >= unspecified | 74.0.1 |
| ThunderbirdMozilla | >= unspecified | 68.7.0 |
| Firefox ESRalpine | >= unspecified | 68.6.1-r0 |
| librewolfalpine | See advisory | 74.0.1-r0 |
| mozjs68alpine | See advisory | 68.7.0-r0 |
Patched Releases
Firefox
0:68.6.1-1.el6_10, 0:68.6.1-1.el7_8, 0:68.6.1-1.el8_0, 68.6.1-1.0.1.el6_10, 68.6.1-1.0.1.el7_8, 68.6.1-1.0.1.el8_1, 74.0.1-1, 74.0.1-r0, 74.0.1+build1-0ubuntu0.16.04.1, 74.0.1+build1-0ubuntu0.18.04.1, 75.0+build3-0ubuntu1, 74.0.1, 68.6.1
Firefox ESR
68.6.1, 128.5.1-1.1, 68.6.1-r0, 68.6.1esr-1, 68.6.1esr-1~deb10u1, 68.6.1esr-1~deb9u1, 68.7.0esr-1~deb8u1
firefox-esr-branding-upstream
128.5.1-1.1
firefox-esr-translations-common
128.5.1-1.1
firefox-esr-translations-other
128.5.1-1.1
librewolf
74.0.1-r0
Thunderbird
68.7.0, 0:68.7.0-1.el6_10, 0:68.7.0-1.el7_8, 0:68.7.0-1.el8_1, 1:68.7.0-1, 1:68.7.0-1~deb10u1, 1:68.7.0-1~deb8u1, 1:68.7.0-1~deb9u1, 1:68.7.0+build1-0ubuntu0.16.04.2, 1:68.7.0+build1-0ubuntu0.18.04.1, 1:68.7.0+build1-0ubuntu1, 68.7.0-1, 68.7.0-1.0.1.el6_10, 68.7.0-1.0.1.el7_8, 68.7.0-1.0.1.el8_1, 68.7.0-r0, 68.8.0-1.amzn2
MozillaFirefox
68.6.1-109.113.1, 68.6.1-3.81.1, 68.6.1-78.67.1, 68.6.1-lp151.2.39.1, 92.0-1.2
MozillaFirefox-branding-upstream
68.6.1-lp151.2.39.1, 92.0-1.2
MozillaFirefox-buildsymbols
68.6.1-lp151.2.39.1
MozillaFirefox-devel
68.6.1-109.113.1, 68.6.1-3.81.1, 68.6.1-lp151.2.39.1, 92.0-1.2
MozillaFirefox-translations-common
68.6.1-109.113.1, 68.6.1-3.81.1, 68.6.1-78.67.1, 68.6.1-lp151.2.39.1, 92.0-1.2
MozillaFirefox-translations-other
68.6.1-3.81.1, 68.6.1-78.67.1, 68.6.1-lp151.2.39.1, 92.0-1.2
MozillaThunderbird
68.7.0-3.77.1, 68.7.0-lp151.2.31.1, 68.7.0-lp151.2.35.1, 91.1.1-1.1
MozillaThunderbird-translations-common
68.7.0-3.77.1, 68.7.0-lp151.2.31.1, 68.7.0-lp151.2.35.1, 91.1.1-1.1
MozillaThunderbird-translations-other
68.7.0-3.77.1, 68.7.0-lp151.2.31.1, 68.7.0-lp151.2.35.1, 91.1.1-1.1
mozjs60
NA
mozjs68
68.7.0-r0
Timeline
Added to Industry KEV feed
Apr 1, 2020
Publicly disclosed
Apr 24, 2020
Added to CISA KEV
Nov 3, 2021
Added to CIRCL KEV
Nov 3, 2021
Added to ENISA EUVD
Nov 3, 2021
Last updated
Aug 5, 2026
Weakness Type
- CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
- CWE-416 Use After Free
References
- advisoryRHSA-2020:1338
- advisoryRHSA-2020:1339
- advisoryRHSA-2020:1340
- advisoryRHSA-2020:1341
- advisoryRHSA-2020:1488
FAQ
Common Questions.
Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.
Affected: Firefox (Mozilla) versions >= unspecified; Thunderbird (Mozilla) versions >= unspecified; Firefox ESR (alpine) versions >= unspecified, among others. The full list is in the CVE brief.
It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Apr 1, 2020.
Yes. It was added on Nov 3, 2021 with a federal remediation due date of May 3, 2022.
Public exploit code is available.
Fixed versions are available, including Firefox 0:68.6.1-1.el6_10, Firefox 0:68.6.1-1.el7_8, Firefox 0:68.6.1-1.el8_0.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
Is CVE-2020-6820 Running On Your Attack Surface?
Brandefense maps vulnerabilities like this one to your internet-facing assets and tells you exactly where to patch.
Request A Demo