What’s Vulnerable › Vulnerabilities

CVE-2021-38509

• Severity: medium (CVSS 4.3 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-1021

Description

Due to an unusual sequence of attacker-controlled events, a Javascript alert() dialog with arbitrary (although unstyled) contents could be displayed over top an uncontrolled webpage of the attacker's choosing. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Affected products

• Debian Linux: 9.0, 10.0, 11.0
• Mozilla Firefox: before 94.0 — fixed in 94.0
• 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 94.0; 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=1718571
• 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/mfsa2021-48/
• https://www.mozilla.org/security/advisories/mfsa2021-49/
• https://www.mozilla.org/security/advisories/mfsa2021-50/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1718571
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html

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