What’s Vulnerable › Vulnerabilities

CVE-2021-23987

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-31 · updated 2026-06-17
• Weakness: CWE-787

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.

Affected products

• Mozilla Firefox: before 87.0 — fixed in 87.0
• Mozilla Firefox ESR: before 78.9 — fixed in 78.9
• Mozilla Thunderbird: before 78.9 — fixed in 78.9

How to fix

Upgrade to a fixed version: Mozilla Firefox 87.0; Mozilla Firefox ESR 78.9; Mozilla Thunderbird 78.9. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718
• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://www.mozilla.org/security/advisories/mfsa2021-11/
• https://www.mozilla.org/security/advisories/mfsa2021-12/
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718
• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://www.mozilla.org/security/advisories/mfsa2021-11/
• https://www.mozilla.org/security/advisories/mfsa2021-12/

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