CVE-2021-43542: Debian Linux

Medium severity, CVSS 6.5. EPSS: 1.8% chance of exploitation in the next 30 days.

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 Debian Linux: version 9.0 only; version 10.0 only; version 11.0 only
• 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)

Published 2021-12-08. Last modified 2026-06-17.