CriticalCVE-2021-1870

    Apple iOS, iPadOS, and macOS WebKit Remote Code Execution Vulnerability (CVE-2021-1870)

    Published In CISA KEV since Sources: CVE.org, Debian, Red Hat

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    What is affected, what to do

    An unauthenticated attacker can exploit a logic vulnerability in Apple WebKit on iOS, iPadOS, and macOS to execute arbitrary code. This affects HTML parsers using WebKit, including Apple Safari and some non-Apple products. The vulnerability is actively exploited, as listed in the CISA KEV catalog since November 2021. Affected versions include up to iOS 14.4, iPadOS 14.4, macOS Big Sur 11.2, and macOS Catalina 10.15.7. Immediate patching to the specified security updates is critical.

    The vulnerability is being actively exploited; treat it as a priority. Update Apple iOS and iPadOS to version 14.4, Apple macOS to version 11.2. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Apple iOS and iPadOS lt *..14.4
    • Apple macOS lt *..11.2
    Fixed in
    • Apple iOS and iPadOS 14.4
    • Apple macOS 11.2

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org, Debian, Red Hat

    7.7 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 94.6

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    220.64 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: Apple (CVE.org) How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    7.7 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    94.6
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 29 January 2025

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication, and an attacker only needs a user to load crafted web content via a messenger or email link or a watering-hole site. Apple acknowledged active exploitation when it released the fix. WebKit bugs like this typically serve as the initial stage of targeted exploit chains against high-value individuals rather than in mass ransomware operations, and CISA lists no known ransomware use. Exploitation likelihood ranks among the higher tier of all CVEs. Because patches have been available for a long time, the real residual risk sits in legacy hardware, unmanaged BYOD and third-party products embedding WebKit. Organisations with fully enforced update compliance can close this out. Anyone still running unpatchable Apple devices for executives or administrators should treat those devices as a high-priority risk and replace them promptly.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Enforce OS updates: Use your MDM (Jamf, Intune, Kandji, etc.) to move all managed Apple devices to at least iOS 14.4 / iPadOS 14.4 or macOS Big Sur 11.2. Deploy Security Update 2021-001 Catalina and Security Update 2021-001 Mojave to legacy macOS systems. Any device on a current release is already fixed, so focus on stragglers and devices without enforced updates.
      • Quantify residual exposure: Pull an MDM report of every device below iOS/iPadOS 14.4 or macOS 11.2. Include unmanaged BYOD devices that reach corporate services. Track hardware that can no longer be updated (end-of-support) as a separate risk class.
      • Block non-compliant devices: Enforce a minimum OS version through Conditional Access or compliance policies for mail, VPN, SSO and M365/Google Workspace. Quarantine non-compliant devices until they are updated.
      • Check third-party WebKit consumers: CISA notes that non-Apple HTML parsers built on WebKit may also be affected, such as WebKitGTK/WPE-based applications and kiosk or embedded systems. Inventory these products. A patch is available from the vendor; look up the fixed version in the vendor advisory.
      • Prioritise high-risk users: Update executives, administrators and frequent travellers first. If compromise is suspected, isolate and image the device rather than wiping it.

    2. Runbook step 2

      Mitigation layers

      • Configuration management (MDM): Enforce updates with deadlines, for example via software update enforcement or Declarative Device Management. Cap update deferrals at a few days.
      • Endpoint hardening: On supervised iOS/iPadOS devices that cannot be updated immediately, disable JavaScript via the Safari restrictions payload ("Allow JavaScript" = false). This removes most of the attack surface typical WebKit exploit chains rely on. Note that every iOS/iPadOS browser (Chrome, Firefox, Edge) and every in-app webview runs on WebKit, so switching browsers is not a mitigation.
      • Network / web filtering: On your secure web gateway or proxy, block uncategorised, newly registered and malware/phishing-classified domains for mobile device groups. Enforce protective DNS on mobile devices through an always-on VPN or a DNS profile.
      • Segmentation: Restrict Apple devices that cannot be updated to a guest or internet-only segment with no path to internal applications, admin interfaces or OT/critical-infrastructure networks.
      • IAM / least privilege: Prohibit privileged accounts (domain admin, cloud admin) on mobile devices running outdated OS versions. Use phishing-resistant MFA (FIDO2) to limit the value of sessions or tokens stolen from a compromised device.

    3. Runbook step 3

      Detection rules

      • Proxy/web logs (User-Agent): Identify outdated devices from their User-Agent strings, e.g. regex CPU (iPhone )?OS 14_[0-3] or OS 1[0-3]_ for iOS/iPadOS. Example SPL: index=proxy http_user_agent="*CPU*OS 14_[0-3]*" | stats count by src_ip, user (adapt the wildcard to a regex in your SIEM). Treat the results as an exposure list and a hunting starting point.
      • EDR process ancestry on macOS: The WebKit renderer does not spawn shells or interpreters in normal operation. KQL (Defender for Endpoint): DeviceProcessEvents | where InitiatingProcessFileName == "com.apple.WebKit.WebContent" and FileName in~ ("sh","bash","zsh","curl","osascript","python3","perl"). Sigma shape: logsource: product: macos, category: process_creation with ParentImage|endswith: '/com.apple.WebKit.WebContent'.
      • Crash telemetry: Clusters of com.apple.WebKit.WebContent crash reports can indicate failed exploitation attempts. Look in ~/Library/Logs/DiagnosticReports/ on macOS and in Analytics data or a sysdiagnose on iOS. Correlate crashes with visits to unfamiliar domains shortly beforehand.
      • Post-exploitation persistence (macOS): Alert on new LaunchAgents/LaunchDaemons and on file writes to ~/Library/LaunchAgents/ or /Library/LaunchDaemons/ that occur close in time to Safari activity (EDR file events or Endpoint Security Framework telemetry).
      • iOS forensics: When compromise is suspected, take a Finder/iTunes backup or sysdiagnose and scan it with the Mobile Verification Toolkit (MVT) against current IOC sets. In Zeek/Suricata, watch for unusual outbound connections from the device group to newly registered domains.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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..

    Source: NVD (NIST), CISA KEV Catalog, cvelistv5, Debian Security Advisories, redhat_csaf

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    AppleiOS and iPadOS
    • < 14.4
    • 14.4
    ApplemacOS
    • < 11.2
    • 11.2
    webkitgtkwebkitgtk
    • < 2.30.6
    • 2.30.6

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianwebkit2gtkbuster
    macOSapplemac_os_x10.15.7
    macOSapplemac_os_x—
    macOSapplemacos—
    Mobileappleiphone_os—
    Otherfedoraprojectfedora32
    Otherfedoraprojectfedora33
    Otherappleipados—

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    Apple (CVE.org)CVE.org (MITRE) (opens in a new tab)actively exploited

    References & sources

    • support.apple.comen-us/HT212147x_refsource_MISC, Release notes, Vendor advisory
    • support.apple.comen-us/HT212146x_refsource_MISC, Release notes, Vendor advisory
    • lists.fedoraproject.orgFEDORA-2021-864dc37032Vendor advisory, x_refsource_FEDORA, Broken link, Mailing list
    • lists.fedoraproject.orgFEDORA-2021-619711d709Vendor advisory, x_refsource_FEDORA, Broken link, Mailing list
    • security.gentoo.orgGLSA-202104-03Vendor advisory, x_refsource_GENTOO, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-1870government-resource, US government resource
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