CVE-2021-38509
MEDIUMEPSS 75th pctlDescription
Due to an unusual sequence of attacker-controlled events, a Javascript alert() dialog with arbitrary (although unstyled) contents could be displayed over top an uncontrolled webpage of the attacker's choosing. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.
How to fix
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
This Ubuntu release reached end-of-life on 2025-05-31. Extended security maintenance is available until 2030-04-02.
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
This Ubuntu release reached end-of-life on 2025-05-31. Extended security maintenance is available until 2030-04-02.
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Exploit Intelligence
Elevated risk: more likely to be exploited than 75% of all known CVEs.
References
Related Vulnerabilities
Other CWE-1021 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2021-21132 | Critical | 9.6 | 23% | - | Fix |
| CVE-2011-1244 | Medium | 5.8 | 15% | - | - |
| CVE-2013-2682 | Medium | 4.3 | 6.3% | - | - |
| CVE-2021-21139 | Medium | 6.5 | 4.7% | - | Fix |
| CVE-2017-11290 | Medium | 6.1 | 3.0% | - | - |
| CVE-2008-2716 | Medium | 5.0 | 3.0% | - | Fix |
Common questions
How do I fix CVE-2021-38509?
Published advisories record a fix for 7 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2021-38509 being actively exploited?
Not that we know of. CVE-2021-38509 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.6% is the estimated probability that it will be exploited in the next 30 days. That is higher than 75% of all scored CVEs.
How severe is CVE-2021-38509?
CVE-2021-38509 has a CVSS v3 base score of 4.3, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2021-38509 affect?
Published advisories record a fix for firefox (Debian), firefox (Ubuntu), thunderbird (Ubuntu), Firefox Developer Edition (en-US) (Windows application), and 3 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
Embed a live status badge for CVE-2021-38509
Markdown
[](https://tridentstack.com/cve/CVE-2021-38509)HTML
<a href="https://tridentstack.com/cve/CVE-2021-38509"><img src="https://tridentstack.com/cve/badge/CVE-2021-38509.svg" alt="CVE-2021-38509"></a>Check your Linux endpoints for this class of vulnerability
TridentStack Control continuously scans Linux endpoints for known vulnerabilities and deploys the fixes from the same console. 200 endpoints free forever, no credit card.
This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-17.