How severe is CVE-2021-1871?

CVE-2021-1871 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-1871?

Affected package information for CVE-2021-1871 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-1871?

No fixed version has been published for CVE-2021-1871 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-1871 actively exploited in the wild?

Yes. There are 4 known exploit references for CVE-2021-1871, including 4 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-1871?

CVE-2021-1871 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 66th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

Is CVE-2021-1871 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-1871 (Apple iOS, iPadOS, and macOS WebKit Remote Code Execution Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03. CISA required all US federal civilian agencies to remediate this vulnerability by 2021-11-17. The required action was: Apply updates per vendor instructions. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

When was CVE-2021-1871 published, and has it been updated?

CVE-2021-1871 was published on April 2, 2021 and was last updated on April 16, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2021-11-03 — agencies required to remediate by 2021-11-17

CVE-2021-1871

CRITICAL

Published

Apr 2, 2021

Updated

Apr 16, 2026

Affected

0 pkgs

Patched

None yet

Exploits

4 known

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk66th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

A logic issue was addressed with improved restrictions. This issue is fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave, iOS 14.4 and iPadOS 14.4. A remote attacker may be able to cause arbitrary code execution. Apple is aware of a report that this issue may have been actively exploited..

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

A logic issue was addressed with improved restrictions. This issue is fi…

support.apple.comAppleMay 2021

A logic issue was addressed with improved restrictions. This issue is fi…

support.apple.comGoogle-sheetJan 2021

A logic issue was addressed with improved restrictions. This issue is fi…

nvd.nist.govNVDMay 2021

A logic issue was addressed with improved restrictions. This issue is fi…

cisa.govCISANov 2021

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-1871 in your stack?

O3 detects CVE-2021-1871 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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