What’s Vulnerable › Vulnerabilities

CVE-2021-43535

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-12-08 · updated 2026-06-17
• Weakness: CWE-416

Description

A use-after-free could have occured when an HTTP2 session object was released on a different thread, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 93, Thunderbird < 91.3, and Firefox ESR < 91.3.

Affected products

• Debian Linux: 9.0, 10.0, 11.0
• Mozilla Firefox: before 93.0 — fixed in 93.0
• Mozilla Firefox ESR: before 91.3.0 — fixed in 91.3.0
• Mozilla Thunderbird: before 91.3.0 — fixed in 91.3.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 93.0; Mozilla Firefox ESR 91.3.0; Mozilla Thunderbird 91.3.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1667102
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html
• https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html
• https://www.debian.org/security/2021/dsa-5026
• https://www.debian.org/security/2022/dsa-5034
• https://www.mozilla.org/security/advisories/mfsa2021-43/
• https://www.mozilla.org/security/advisories/mfsa2021-49/
• https://www.mozilla.org/security/advisories/mfsa2021-50/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1667102
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html
• https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html
• https://www.debian.org/security/2021/dsa-5026

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