CVE-2026-84121: Sandbox escape due to use-after-free in the DOM: Security component
- Is it exploited?
Not on CISA's list. CISA has not added it to the Known Exploited Vulnerabilities catalog as of . If CISA adds it, this page shows it as known exploited.
- How likely is exploitation in the next 30 days?
- 0.3%
EPSS score, percentile 24.9 of all scored records. A probability estimated by FIRST, not a report of exploitation.
- How severe?
- 9.6 Critical
CVSS v3.1 base score, from CISA's enrichment of the record
- What should I do?
CISA has set no required action, because the vulnerability is not in KEV.
The CVE record links no vendor advisory, patch or mitigation.
- Which versions are affected?
Unaffected 115.40; 140.15; 153.2; 155; 140.15; 153.2; and 1 more
Source: CISA KEV 2026.10.28, as of ; FIRST EPSS, as of ; CVE List, as of . About the data.
What to do
Advisories and patches
The CVE record links no vendor advisory, patch or mitigation.
This summary is based on the CVE record as of . Check the vendor's advisory before you act: affected versions and fixes can change.
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Unaffected | 115.40 |
| Unaffected | 140.15 |
| Unaffected | 153.2 |
| Unaffected | 155 |
| Status | Versions |
|---|---|
| Unaffected | 140.15 |
| Unaffected | 153.2 |
| Unaffected | 155 |
Versions the record lists as unaffected: 115.40, 140.15, 153.2, 155.
Source: CVE List, as of . About the data.
About this vulnerability
Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
Summary: CVE record, as of .
- CISA has not added CVE-2026-84121 to its Known Exploited Vulnerabilities (KEV) catalog, as of 28 Oct 2026.
- The CVE record was published on 1 Sep 2026.
- The CVE record carries a CVSS v3.1 base score of 9.6 (Critical), taken from CISA's enrichment of the record.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 0.3% (percentile 24.9), as of 29 Oct 2026. EPSS is a statistical estimate, not a report of exploitation.
- CISA's SSVC assessment in the CVE record: exploitation none, automatable no, technical impact total.
Source: CISA KEV 2026.10.28, as of ; CVE List, as of ; FIRST EPSS, as of . About the data.
Description
Sandbox escape due to use-after-free in the DOM: Security component
Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
Source: CVE List, as of . About the data.
Weakness
CWE-416
- CWE-416: Use After Free
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record.
Timeline
| Date | Event |
|---|---|
| CVE record published | |
| CVE record last updated |
References
8
Known exploited Mozilla vulnerabilities
- CVE-2010-3765: Mozilla Multiple Products Remote Code Execution Vulnerability
- CVE-2024-9680: Mozilla Firefox Use-After-Free Vulnerability
- CVE-2016-9079: Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free Vulnerability
- CVE-2015-4495: Mozilla Firefox Security Feature Bypass Vulnerability
- CVE-2019-11707: Mozilla Firefox and Thunderbird Type Confusion Vulnerability