CVE-2021-38509 Cross-site scripting in Firefox

What is CVE-2021-38509?

CVE-2021-38509 is a medium-severity cross-site scripting vulnerability in Debian Debian Linux, published on 8 Dec 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official 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.

Weakness: CWE-1021

Which versions are affected by CVE-2021-38509?

It affects 4 products.

Products include Debian Debian Linux, Mozilla Firefox, Mozilla Firefox Esr, Mozilla Thunderbird.

How do I fix CVE-2021-38509?

Upgrade to a fixed version.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is CVE-2021-38509 being exploited?

There is no known exploitation of CVE-2021-38509 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-38509, GHSA-hw7m-5984-jp2q, UBUNTU-CVE-2021-38509.