CVE-2021-1871
Apple iOS, iPadOS, and macOS WebKit Remote Code Execution Vulnerability
Description
CVE-2021-1871 is a critical-severity logic vulnerability in Apple WebKit affecting iOS, iPadOS, and macOS. The flaw allows a remote attacker to cause arbitrary code execution through a logic issue in WebKit's restriction mechanisms. CISA has added this vulnerability to its Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation. With a CVSS score of 9.8 and an EPSS score of 0.86% (74.7th percentile), CVE-2021-1871 poses a maximum-severity remote code execution threat across Apple platforms and Linux distributions using WebKit.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| apple | ipados | < 14.4 |
| apple | iphone os | < 14.4 |
| apple | mac os x | >= 10.15, < 10.15.7; 10.15.7 |
| apple | macos | >= 11.0.1, < 11.2 |
| debian | debian linux | 10.0 |
| fedoraproject | fedora | 33 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/L3L6ZZOU5JS7E3RFYGLP7UFLXCG7TNLU/(Broken Link, Mailing List)
- https://support.apple.com/en-us/HT212146(Release Notes, Vendor Advisory)
- https://support.apple.com/en-us/HT212147(Release Notes, Vendor Advisory)
- https://www.debian.org/security/2021/dsa-4923(Mailing List, Third Party Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-1871(US Government Resource)
Weakness Type
Logic Issue in WebKit
This vulnerability is caused by a logic flaw in WebKit's restriction handling that was not assigned a specific CWE by NVD. Logic issues allow unintended program behavior by exploiting flaws in the application's decision-making processes. In CVE-2021-1871, the insufficient restrictions in WebKit enable remote attackers to execute arbitrary code through crafted web content without any user interaction.
Learn more: NVD — CVE-2021-1871
Impact Analysis
CVE-2021-1871 carries a CVSS 3.1 score of 9.8 (CRITICAL), indicating a maximum-severity threat. The vulnerability is remotely exploitable over the network with low attack complexity and requires neither authentication nor user interaction, representing the most dangerous exploitability profile possible. Successful exploitation results in high impact to confidentiality, integrity, and availability, allowing the attacker to execute arbitrary code on the target system. The EPSS score of 0.86% (74.7th percentile) indicates a notable probability of exploitation. Apple has acknowledged reports that this issue may have been actively exploited, and the vulnerability also affects Debian Linux and Fedora distributions through WebKit dependencies.
Exploit Maturity
CVE-2021-1871 is confirmed as actively exploited through its inclusion in the CISA Known Exploited Vulnerabilities catalog. Apple has acknowledged that this issue may have been actively exploited in the wild. The EPSS score of 0.86% (74.7th percentile) indicates a meaningful exploitation probability. No dedicated public exploit code was identified in the available references. Federal agencies were required to remediate by 2021-11-17 per CISA's binding operational directive. This vulnerability shares the same root cause as CVE-2021-1870, both being logic issues in WebKit's restriction handling disclosed and patched simultaneously.
Remediation
- Apply vendor patches immediately as mandated by CISA KEV: Apply updates per vendor instructions. Update to iOS 14.4/iPadOS 14.4 and macOS Big Sur 11.2, or apply Security Update 2021-001 Catalina/Mojave for older macOS.
- Debian Linux 10.0 users should apply the relevant security advisory (DSA-4923) to update WebKit packages. Fedora 33 users should apply all available security updates.
- Deploy network-level web content filtering and DNS-based threat prevention to reduce exposure to malicious web content that may exploit this WebKit logic vulnerability.
- Monitor WebKit-based browser and application processes for anomalous behavior, including unexpected code execution, unusual network activity, or signs of sandbox escape.
- For high-value targets, consider implementing browser isolation solutions to contain potential WebKit exploitation within an isolated environment, preventing lateral movement to the host system.
Technical Details
CVE-2021-1871 is a logic vulnerability in Apple's WebKit rendering engine caused by insufficient restrictions in the engine's processing logic. The CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) reflects the most exploitable configuration: network-accessible, low complexity, no privileges required, and no user interaction needed. The logic flaw allows a remote attacker to bypass WebKit's intended restrictions and achieve arbitrary code execution. This vulnerability was disclosed and patched simultaneously with CVE-2021-1870, both addressing related logic issues in WebKit. Apple fixed the vulnerability with improved restrictions in macOS Big Sur 11.2, iOS 14.4, and iPadOS 14.4, while Debian addressed it through DSA-4923.
Frequently Asked Questions
Is CVE-2021-1871 being actively exploited?
Yes. CVE-2021-1871 is listed in the CISA Known Exploited Vulnerabilities catalog, and Apple has acknowledged reports of active exploitation. The EPSS score of 0.86% (74.7th percentile) indicates notable exploitation probability.
What products are affected by CVE-2021-1871?
CVE-2021-1871 affects Apple iPhone OS (< 14.4), iPadOS (< 14.4), Mac OS X (>= 10.15, < 10.15.7; 10.15.7), macOS (>= 11.0.1, < 11.2), Debian Linux (10.0), and Fedora (33).
How do I fix CVE-2021-1871?
Apply updates per vendor instructions. Update Apple devices to iOS 14.4, iPadOS 14.4, or macOS Big Sur 11.2. For older macOS, apply Security Update 2021-001. Debian users should apply DSA-4923, and Fedora users should install available security updates.
How severe is CVE-2021-1871?
CVE-2021-1871 has a CVSS 3.1 score of 9.8 (CRITICAL). The vulnerability requires no authentication and no user interaction for remote exploitation, making it one of the most severe WebKit vulnerabilities with confirmed active exploitation.
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