Mozilla · Firefox and Thunderbird
CVE-2020-6820
Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
What it is
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.
CISA: Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.
What to do
Apply updates per vendor instructions.
Deadline for US federal agencies: May 3, 2022
More exploited flaws from Mozilla
| Added | Vulnerability | Product | Severity | Attack chance |
|---|---|---|---|---|
| May 23, 2022 | Type Confusion | Firefox and Thunderbird | High 8.8 | 38% |
| May 23, 2022 | Sandbox Escape | Firefox and Thunderbird | Critical 10.0 | 56% |
| Mar 28, 2022 | Denial-of-Service | Firefox and Thunderbird | High 8.8 | 69% |
| Nov 3, 2021 | Use-After-Free | Firefox and Thunderbird | High 8.1 | 3.0% |
| Nov 3, 2021 | Type Confusion | Firefox and Thunderbird | High 8.8 | 46% |
| Oct 6, 2025 | Remote Code Execution | Multiple Products | Critical 9.8 | 83% |
| Oct 15, 2024 | Use-After-Free | Firefox | Critical 9.8 | 23% |
| Jun 22, 2023 | Use-After-Free | Firefox, Firefox ESR, and Thunderbird | High 7.5 | 87% |
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.