What’s Vulnerable › Vulnerabilities

CVE-2021-43542

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% 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-209

Description

Using XMLHttpRequest, an attacker could have identified installed applications by probing error messages for loading external protocols. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and Firefox < 95.

Affected products

• Debian Linux: 9.0, 10.0, 11.0
• Mozilla Firefox: before 95.0 — fixed in 95.0
• Mozilla Firefox ESR: before 91.4.0 — fixed in 91.4.0
• Mozilla Thunderbird: before 91.4.0 — fixed in 91.4.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 95.0; Mozilla Firefox ESR 91.4.0; Mozilla Thunderbird 91.4.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1723281
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html
• https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html
• https://security.gentoo.org/glsa/202202-03
• https://security.gentoo.org/glsa/202208-14
• https://www.debian.org/security/2021/dsa-5026
• https://www.debian.org/security/2022/dsa-5034
• https://www.mozilla.org/security/advisories/mfsa2021-52/
• https://www.mozilla.org/security/advisories/mfsa2021-53/
• https://www.mozilla.org/security/advisories/mfsa2021-54/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1723281
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html

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