Exploited vulnerability · CISA KEV

Is CVE-2021-21166 exploited? Yes: in CISA KEV since 3 Nov 2021

Gociux · Google Chromium · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2021-21166, Google Chromium Race Condition Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 3 Nov 2021, 239 days after it was published. It affects Google Chromium. If you run it, CISA's required action is: Apply updates per vendor instructions.

exploitedYesCISA KEV · 3 Nov 2021

ransomwareNot knownno confirmed use

EPSStop 5%very likely to be exploited

severity8.8CVSS 3.1 · high

US federal due17 Nov 2021BOD 22-01 deadline

What it is

Google Chromium contains a race condition vulnerability that allows a remote attacker to potentially exploit heap corruption via a crafted HTML page. This vulnerability could affect multiple web browsers that utilize Chromium, including, but not limited to, Google Chrome, Microsoft Edge, and Opera.

Vendor	Google (77 exploited CVEs in KEV)
Product	Chromium
Vulnerability	Google Chromium Race Condition Vulnerability
Weakness	CWE-122, CWE-362
Published	9 Mar 2021
Added to KEV	3 Nov 2021
Reported by (CNA)	Chrome

What to do

Check whether you run it. Look for Chromium in your asset inventory, including appliances and cloud images. Stack check lists every exploited Google CVE at once.
Apply the fix. Apply updates per vendor instructions.
If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 9 Mar 2021.
Get told next time. Email alerts when CISA adds a CVE for Google.

Other exploited CVEs in Chromium

CVE-2026-125350: Google Chromium Use-After-Free Vulnerability (10 Nov 2026)
CVE-2026-2441: Google Chromium CSS Use-After-Free Vulnerability (17 Feb 2026)
CVE-2025-14174: Google Chromium Out of Bounds Memory Access Vulnerability (12 Dec 2025)
CVE-2025-6558: Google Chromium ANGLE and GPU Improper Input Validation Vulnerability (22 Jul 2025)
CVE-2024-4671: Google Chromium Visuals Use-After-Free Vulnerability (13 May 2024)
CVE-2021-37976: Google Chromium Information Disclosure Vulnerability (3 Nov 2021)

References

nvd.nist.gov/vuln/detail/CVE-2021-21166

Questions

Is CVE-2021-21166 being exploited?

Yes. CISA added CVE-2021-21166 to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, which means there is reliable evidence of exploitation in the wild.

Is CVE-2021-21166 used in ransomware attacks?

CISA does not list known ransomware use for it. That can change, and exploitation alone is reason to fix it first.

How do I fix CVE-2021-21166?

Apply updates per vendor instructions. Check Google's advisory for the fixed versions, and if you can't patch quickly, restrict access to the affected Chromium and watch it for signs of compromise.

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in November 2021 · Google scorecard · act-now radar.

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