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 release is recorded for CVE-2021-1871 yet. Track the upstream advisory, and apply the exposure-reduction steps below in the meantime.

How do I detect CVE-2021-1871 in my code and dependencies?

Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

How do I mitigate CVE-2021-1871 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

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 7.0%, placing it in the 94th 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 · ACTIVELY EXPLOITED

CRITICAL severity

CVE-2021-1871

CRITICAL

CVE-2021-1871 is a critical-severity (CVSS 9.8) remote code execution vulnerability. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.

Published

Apr 2, 2021

Updated

Apr 16, 2026

Affected

6 products

Patched

See advisory

Exploits

4 known

Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, CISA KEV, FIRST.org (EPSS)

Exploitation Status

Actively exploited in the wild

Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-11-03. Federal agencies were required to remediate by 2021-11-17.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-1871.

EPSS Exploitation Probability

via FIRST.org ↗

7.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs94th percentile — riskier than 94% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-1871 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 395,249 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

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..

Affected Products

6 products · 9 configurations

ipadosapple

< 14.4

range

iphone osapple

< 14.4

range

mac os xapple

≥ 10.15 && < 10.15.7

1 version

10.15.7

macosapple

≥ 11.0.1 && < 11.2

range

debian linuxdebian

1 version

10.0

fedorafedoraproject

1 version

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

Detection & mitigation playbook

Vulnerability

Detect
Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).
Remediation status
No fixed release is recorded for CVE-2021-1871 yet. Track the upstream advisory, and apply the exposure-reduction steps below in the meantime.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This flaw is being rated as 'Moderate' as WebKitGTK package is shipped as a dependency for Gnome package, additionally Red Hat Enterprise Linux doesn't ship any WebKitGTK based web browser where this flaw would present a higher severity major threat.

Product	Fixed in	Advisory
Red Hat Enterprise Linux 7 Extended Lifecycle Support	`webkitgtk4-0:2.48.3-2.el7_9`	RHSA-2025:10364
Red Hat Enterprise Linux 8	`webkit2gtk3-0:2.32.3-2.el8`	RHSA-2021:4381

Frequently Asked Questions

O3 Security · Impact-Aware SCA

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CVE-2021-1871

Exploitation Status

Actively exploited in the wild

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Description

Affected Products

Detection & mitigation playbook

Detect

Remediation status

Mitigate without a patch

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-1871 in your dependencies?