What is CVE-2019-9821?

CVE-2019-9821 is a high-severity vulnerability in Mozilla Firefox, classified under Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). CVSS score: 8.1/10. Published 2019-07-23.

How severe is CVE-2019-9821?

High severity. CVSS v3 base score is 8.1 out of 10.

Use After Free in Mozilla Firefox

CVE-2019-9821

A use-after-free vulnerability can occur in AssertWorkerThread due to a race condition with shared workers. This results in a potentially exploitable crash. This vulnerability affects Firefox < 67.

Vulnerability class: Race Condition

Published 2019-07-23 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.1 (High). Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.008 (56.6th percentile), scored 2026-10-06.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 57th percentile — 56.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.008 · change: -0.000

Affected products

Mozilla Firefox — versions unspecified

Weakness classification (CWE)

References

www.mozilla.org/security/advisories/mfsa2019-13/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Vendor Advisory)

Frequently asked questions

What is CVE-2019-9821?: CVE-2019-9821 is a high-severity vulnerability in Mozilla Firefox, classified under Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). CVSS score: 8.1/10. Published 2019-07-23.
How severe is CVE-2019-9821?: High severity. CVSS v3 base score is 8.1 out of 10.