+44 1865 950 828 Secure Your Business
Actively exploited

CVE-2021-30663 Apple Multiple Products WebKit Integer Overflow Vulnerability

Apple iOS, iPadOS, macOS, tvOS, and Safari WebKit contain an integer overflow vulnerability that leads to code execution when processing maliciously crafted web content. This vulnerability could impact HTML parsers that use WebKit, including but not limited to Apple Safari and non-Apple products which rely on WebKit for HTML processing.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 3 Nov 2021.
  • Rated High severity (CVSS 7.8, scored by CISA).
  • It has been in the catalogue for 1,829 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

An integer overflow was addressed with improved input validation. This issue is fixed in iOS 14.5.1 and iPadOS 14.5.1, tvOS 14.6, iOS 12.5.3, Safari 14.1.1, macOS Big Sur 11.3.1. Processing maliciously crafted web content may lead to arbitrary code execution.

Affected Versions

ProductVersions
Apple macOSunspecified to before 11.3
Apple macOSunspecified to before 14.5
Apple macOSunspecified to before 12.5
Apple macOSunspecified to before 14.6
Apple macOSunspecified to before 14.1

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Apple Multiple Products. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 17 Nov 2021 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Apple Flaws

Get alerts for AppleWe email you when a new Apple vulnerability is confirmed as exploited. Add more vendors on Threat Watch.

By signing up you agree to receive Threat Watch alerts. Unsubscribe at any time. See our Privacy Policy.

CVE-2024-27834Apple Multiple Products Improper Authentication VulnerabilityAdded 4 Nov 2026CVE-2026-86950Apple Multiple Products Out-of-Bounds Write VulnerabilityAdded 29 Sep 2026CVE-2026-65400Apple macOS Improper Authentication VulnerabilityAdded 18 Aug 2026CVE-2025-43510Apple Multiple Products Improper Locking VulnerabilityAdded 20 Mar 2026CVE-2025-43520Apple Multiple Products Classic Buffer Overflow VulnerabilityAdded 20 Mar 2026CVE-2025-31277Apple Multiple Products Buffer Overflow VulnerabilityAdded 20 Mar 2026

All 96 exploited Apple vulnerabilities

Not Sure If You Are Exposed?

Our attack surface and vulnerability teams can check your internet-facing estate for CVE-2021-30663 and every other exploited flaw on the list.

Check My Exposure
Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-30663
  3. CVE record: CVE-2021-30663
  4. FIRST EPSS
  5. support.apple.com/en-us/HT212532
  6. support.apple.com/en-us/HT212534
  7. support.apple.com/en-us/HT212341
  8. support.apple.com/en-us/HT212335
  9. support.apple.com/en-us/HT212336