What’s Vulnerable › Vulnerabilities

CVE-2021-23995

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-06-24 · updated 2026-06-17
• Weakness: CWE-672

Description

When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

Affected products

• Mozilla Firefox: before 88.0 — fixed in 88.0
• Mozilla Firefox ESR: before 78.10 — fixed in 78.10
• Mozilla Thunderbird: before 78.10 — fixed in 78.10

How to fix

Upgrade to a fixed version: Mozilla Firefox 88.0; Mozilla Firefox ESR 78.10; Mozilla Thunderbird 78.10. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1699835
• https://www.mozilla.org/security/advisories/mfsa2021-14/
• https://www.mozilla.org/security/advisories/mfsa2021-15/
• https://www.mozilla.org/security/advisories/mfsa2021-16/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1699835
• https://www.mozilla.org/security/advisories/mfsa2021-14/
• https://www.mozilla.org/security/advisories/mfsa2021-15/
• https://www.mozilla.org/security/advisories/mfsa2021-16/

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