Yes. CISA added CVE-2021-4102 to its Known Exploited Vulnerabilities catalogue on 15 December 2021, which it does only when there is reliable evidence of active exploitation. CISA has not linked it to known ransomware campaigns. Google Chromium V8 Engine contains a use-after-free vulnerability that allows a remote attacker to potentially exploit heap corruption via a crafted HTML page.
What makes this CVE worth treating as actively exploited?
CVE-2021-4102 is not just a theoretical browser weakness. Once a vulnerability appears in the CISA KEV catalogue, the working assumption should shift from “patch when convenient” to “confirm exposure and treat as urgent,” because KEV inclusion signals reliable evidence of exploitation in the wild. For browser flaws, that matters most where users routinely open untrusted content.
That urgency is reinforced by the weakness type itself. A use-after-free in a browser engine can be weaponised through crafted content, so the practical question is less about the abstract bug class and more about whether the affected browser surface is still reachable in your environment. If the browser is internet-facing through normal user activity, exposure is usually broad.
How practitioners should confirm and reduce exposure
Start by identifying where Chromium-based browsers are installed and which endpoints can still reach the vulnerable code path through normal browsing. For this kind of issue, inventory matters as much as patching, because unmanaged browsers, dormant laptops, shared workstations and virtual desktop pools often lag behind centrally managed fleets. The most useful source of version and remediation detail is the vendor advisory, while the CISA Known Exploited Vulnerabilities Catalog gives the prioritisation signal.
Where browser governance is already mature, the next step is to verify whether policy enforcement actually reaches the endpoints that matter. The Enterprise Browser and Browser Security Guide is useful here because browser hardening, isolation and extension control only help if the managed browser is the one people truly use. A second useful control point is Vulnerability Management Guide, which supports the operational side of KEV-driven prioritisation, exception handling and verification.
What to do if patching must wait
If immediate patching is not possible, reduce the chance that a malicious page can reach the vulnerable code path. In practice that means tightening browser isolation, limiting risky browsing profiles, constraining extension installation, and reducing the set of endpoints allowed to browse unsandboxed or under elevated user context. Temporary mitigations are only acceptable when they are specific, documented and actively monitored.
For teams that need a structured way to decide what gets remediated first, browser exploitation should be ranked alongside any other KEV item that has an external attack path and no compensating control of equal strength. That is exactly the sort of triage logic captured in Zero-Day and Exploited Vulnerabilities Guide. Where browser exposure is paired with user-driven content delivery, the CWE Top 25 Guide also helps frame why memory-safety weaknesses remain a recurring exploitation path.
Risk and Threat Considerations
The main risk is that a browser engine flaw becomes an entry point for drive-by compromise. Because the trigger is crafted web content, defenders often underestimate how widely the weakness is exposed, especially in organisations that depend on routine web access for normal business activity.
Failure mechanism: A use-after-free can allow an attacker to corrupt heap state through maliciously constructed content, turning ordinary browsing into code execution or crash conditions if the vulnerable version remains reachable.
Impact: Successful exploitation can lead to endpoint compromise, credential theft, session hijacking, malicious payload delivery or a foothold for follow-on movement from a user workstation.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
MITRE ATT&CK addresses the attack and risk surface, while CIS Controls v8, NIST CSF 2.0, NIST SP 800-53 Rev 5 and OWASP ASVS set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| CIS Controls v8 | CIS-7 — Continuous Vulnerability Management | KEV-listed browser flaws require rapid identification and remediation of exposed assets. |
| Recommendation — Prioritise and remediate KEV-listed browser exposures through continuous vulnerability management. | ||
| NIST CSF 2.0 | GV.RM-01 — Risk Management Strategy | KEV exploitation changes patch urgency and exception handling for exposed endpoints. |
| Recommendation — Treat KEV status as a risk input and accelerate remediation decisions accordingly. | ||
| NIST SP 800-53 Rev 5 | SI-2 — Flaw Remediation | Browser use-after-free vulnerabilities require timely remediation of software flaws. |
| Recommendation — Patch or mitigate the vulnerable browser engine without delay. | ||
| MITRE ATT&CK | T1189 — Drive-by Compromise | Crafted web content is a common delivery path for exploited browser flaws. |
| Recommendation — Monitor for drive-by delivery patterns and web-triggered compromise attempts. | ||
| OWASP ASVS | V12 — Cryptography | Browser exploitation is not primarily an app crypto issue, so this was omitted. |
| Recommendation — Omit this mapping. | ||
Practitioner Guidance
What to verify: Confirm the exact browser build in use on managed and unmanaged endpoints, then verify that the vulnerable browser is not reachable through shadow IT installs, portable browsers or stale virtual desktop images.
Decision rule: If the browser instance can reach untrusted web content and the device cannot be patched immediately, treat the exposure as active until the system is remediated or the browsing surface is materially constrained.
Practitioner takeaway: For KEV-listed browser flaws, the operational question is not whether exploitation is possible in theory, it is whether any reachable endpoint still allows the vulnerable code path to be triggered before you have removed or contained it.