CVE-2019-18854 in safe-svg
Summary
by MITRE
A Denial Of Service vulnerability exists in the safe-svg (aka Safe SVG) plugin through 1.9.4 for WordPress, related to unlimited recursion for a '<use ... xlink:href="#identifier">' substring.
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Analysis
by VulDB Data Team • 02/11/2024
The vulnerability identified as CVE-2019-18854 represents a critical denial of service flaw within the safe-svg WordPress plugin version 1.9.4 and earlier. This issue stems from improper handling of SVG elements containing xlink:href attributes that reference internal identifiers, creating a condition where recursive processing can occur without bounds. The vulnerability specifically targets the plugin's SVG sanitization mechanism, which is designed to prevent malicious SVG content from being executed on WordPress sites. When the plugin encounters an SVG with a <use ... xlink:href="#identifier"> construct that references itself or creates a circular dependency, it fails to implement adequate recursion detection or limit mechanisms.
The technical implementation of this vulnerability exploits the fundamental parsing behavior of SVG elements within the plugin's sanitization process. The safe-svg plugin is intended to allow safe SVG uploads while preventing potential XSS and other security exploits by sanitizing SVG content before rendering. However, the flaw occurs during the parsing phase where the plugin recursively processes xlink:href references without maintaining proper state tracking or depth limitations. This recursive processing continues indefinitely until system resources are exhausted, leading to complete service disruption for the affected WordPress installation. The vulnerability manifests when an attacker uploads or injects an SVG file containing self-referencing or cyclic xlink:href attributes that cause the plugin's internal processing loop to continue without termination.
The operational impact of this vulnerability extends beyond simple service disruption to encompass complete system availability compromise. When exploited, the vulnerability can cause web server processes to consume excessive CPU cycles and memory resources, potentially leading to server crashes or denial of service for legitimate users. Attackers can leverage this vulnerability by uploading malicious SVG files containing recursive xlink:href references, which then trigger the infinite recursion during plugin processing. The attack vector is particularly concerning because it can be executed through normal user upload mechanisms or via other attack vectors that allow SVG content injection, making it difficult to prevent through traditional security measures. This vulnerability affects all WordPress installations using the vulnerable safe-svg plugin version, regardless of hosting environment or server configuration.
Mitigation strategies for CVE-2019-18854 require immediate attention and multiple layers of protection. The primary and most effective solution involves updating the safe-svg plugin to version 1.9.5 or later, which includes proper recursion detection and limit mechanisms. System administrators should also implement additional protective measures such as restricting SVG file uploads to trusted users only, implementing content security policies that limit external resource loading, and monitoring for unusual processing patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-674, which describes "Uncontrolled Recursion" in software systems, and represents a classic example of how insufficient input validation can lead to resource exhaustion attacks. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers "Endpoint Denial of Service" through resource exhaustion, demonstrating how a seemingly minor parsing flaw can be weaponized for significant operational disruption. Organizations should also consider implementing automated monitoring for recursive processing patterns and establishing incident response procedures specifically designed to handle denial of service attacks targeting content management systems.