CriticalCVE-2021-1871

    Apple iOS, iPadOS, and macOS WebKit Remote Code Execution Vulnerability (CVE-2021-1871)

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

    Share

    What is affected, what to do

    This vulnerability allows remote code execution (RCE) in WebKit-based HTML parsers, including Apple Safari and non-Apple products relying on WebKit. An attacker can execute arbitrary code remotely, leading to severe security breaches. The flaw has been actively exploited since November 2021, as confirmed by CISA. Affected products include Apple iOS, iPadOS, and macOS up to version 14.4 and macOS Big Sur up to 11.2. Immediate patching to the specified security updates is critical to mitigate this vulnerability.

    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: Debian, CVE.org, Red Hat

    7.0 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 94.2

    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
    219.96 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
    7.0 %
    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
    94.2
    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 is a WebKit logic flaw that gives an attacker remote code execution without authentication. Apple flagged likely in-the-wild exploitation at release, typically via crafted web content in targeted campaigns against the mobile devices of high-profile users. CISA reports no known ransomware use, and exploitation likelihood sits in the upper range. Because fixes have shipped since early 2021, the residual risk lies almost entirely in unmanaged, forgotten or no-longer-updatable devices, which makes this a governance gap more than a patching task. Rate any unpatched device with access to corporate data as critical and block it; for fully managed fleets, a compliance verification is sufficient.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the fixes: iOS 14.4 / iPadOS 14.4, macOS Big Sur 11.2, Security Update 2021-001 Catalina (10.15.7) and Security Update 2021-001 Mojave. Any later supported OS release also contains the fix. Move devices to the current supported major version rather than the bare minimum.
      • Pull an MDM inventory (Intune, Jamf, Kandji, Workspace ONE): Flag every device below iOS/iPadOS 14.4, or macOS below 11.2 without Security Update 2021-001. Hardware that cannot reach these builds is end-of-life and should be retired.
      • Enforce conditional access: Block non-compliant Apple devices from mail, VPN, SSO and M365/Google Workspace now, e.g. with an Intune "Minimum OS version" compliance policy combined with Entra ID Conditional Access.
      • Check non-Apple WebKit consumers: Inventory appliances, kiosks, embedded HMIs and Linux apps that render HTML via WebKit. A patch is available from each vendor — look up the fixed version in the vendor advisory.
      • Prioritise high-value users: Patch devices belonging to executives, admins, finance/procurement and OT engineering first. If an unpatched device in this group turns up, revoke its session tokens and preserve the device for forensics.

    2. Runbook step 2

      Mitigation layers

      • Secure web gateway: Block uncategorised and newly observed domains for Apple user agents. Proxy rule: a user agent matching iPhone OS 14_[0-3] or Mac OS X 10_15_7 with an outdated Safari build → deny, or redirect to a remediation page.
      • Mail and messaging protection: Turn on URL rewriting and sandbox detonation for inbound links. The typical vector is a crafted link that lures the victim to a malicious page.
      • Endpoint hardening (macOS): Until patched, set a browser with its own rendering engine (Chrome, Firefox) as the default and restrict Safari via MDM. This does not help on iOS/iPadOS, where every browser uses WebKit, so updating or blocking the device are the only real options there.
      • Configuration profile (supervised iOS): Disable the "JavaScript in Safari" restriction to shrink the attack surface. Treat this as a stopgap only, since it breaks site functionality.
      • Network segmentation: Place BYOD and unmanaged Apple devices in a guest VLAN with no route to internal services, management networks or OT zones.
      • Least privilege: Do not give standard macOS users local admin rights. This makes post-exploitation persistence from the WebContent process harder.

    3. Runbook step 3

      Detection rules

      • EDR process ancestry (macOS): Any child process of com.apple.WebKit.WebContent or Safari is highly suspicious. Example KQL: DeviceProcessEvents | where InitiatingProcessFileName has "com.apple.WebKit.WebContent" and FileName in~ ("sh","bash","zsh","curl","osascript","python3","perl")
      • Crash telemetry: Clusters of WebContent crashes in ~/Library/Logs/DiagnosticReports/*.ips (macOS), or in iOS analytics collected via MDM, shortly after users open external links can indicate failed exploit attempts.
      • Proxy / Zeek logs: Look for outdated Apple user agents reaching rare domains, followed by anomalous outbound traffic (beaconing, uploads) from the same host. Sigma shape: logsource: proxy with c-useragent|contains: 'iPhone OS 14_3' and cs-host not in a top-1000 allowlist.
      • Persistence (macOS, ESF/EDR file events): New files under ~/Library/LaunchAgents/ or /Library/LaunchDaemons/ created during or right after a Safari session (correlate with the Unified Log).
      • MDM compliance alerts: Treat any below-baseline device that is still requesting corporate resources as an incident ticket, not a hygiene note.

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

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebiandebian_linux10.0
    Linuxdebianwebkit2gtkbuster
    macOSapplemac_os_x10.15.7
    macOSapplemac_os_x—
    macOSapplemacos—
    Mobileappleiphone_os—
    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
    Apple (CVE.org)CVE.org (MITRE) (opens in a new tab)actively exploited
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively 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
    • debian.orgDSA-4923Vendor advisory, x_refsource_DEBIAN, Mailing list, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-1871government-resource, US government resource
    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2021-1871
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-1871"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-1871.svg?lang=en" alt="CVE-2021-1871 – NEOSEC Intel" height="20"></a>