CVE-2021-23987
Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (1.42%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
Mozilla developers and community members reported memory safety bugs present in Firefox 86 and Firefox ESR 78.8. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.
- CVSS 3.1
- 8.8 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- EPSS
- 1.42% probability · 73th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-787
- Affected
- mozilla/firefox · mozilla/firefox esr · mozilla/thunderbird
- Source
- security@mozilla.org
EPSS 1.4% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
References
- https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718Issue Tracking, Patch, Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-10/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-11/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-12/Vendor Advisory
- https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718Issue Tracking, Patch, Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-10/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-11/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2021-12/Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.