CVE-2021-23999
HIGHDescription
If a Blob URL was loaded through some unusual user interaction, it could have been loaded by the System Principal and granted additional privileges that should not be granted to web content. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
How to fix
This Ubuntu release reached end-of-life on 2021-04-02.
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.
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.
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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:H/I:H/A:H
Exploit Intelligence
Moderate risk: more likely to be exploited than 68% of all known CVEs.
References
Related Vulnerabilities
Other CWE-269 (Improper Privilege Management) vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2017-12635 | Critical | 9.8 | 100% | - | Fix |
| CVE-2022-24637 | Critical | 9.8 | 99% | - | Fix |
| CVE-2017-5689 | Critical | 9.8 | 92% | KEV | Fix |
| CVE-2020-3243 | Critical | 9.8 | 88% | - | - |
| CVE-2024-21888 | High | 8.8 | 87% | - | - |
| CVE-2022-0441 | Critical | 9.8 | 85% | - | Fix |
Common questions
How do I fix CVE-2021-23999?
Published advisories record a fix for 9 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-2021-23999 being actively exploited?
Not that we know of. CVE-2021-23999 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.3% is the estimated probability that it will be exploited in the next 30 days. That is higher than 68% of all scored CVEs.
How severe is CVE-2021-23999?
CVE-2021-23999 has a CVSS v3 base score of 8.8, rated high. 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-2021-23999 affect?
Published advisories record a fix for thunderbird (Amazon), thunderbird-debuginfo (Amazon), firefox (Debian), firefox (Ubuntu), and 5 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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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.