CVE-2020-13756 in PHP CSS Parser
Summary
by MITRE
Sabberworm PHP CSS Parser before 8.3.1 calls eval on uncontrolled data, possibly leading to remote code execution if the function allSelectors() or getSelectorsBySpecificity() is called with input from an attacker.
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Analysis
by VulDB Data Team • 11/03/2025
The vulnerability identified as CVE-2020-13756 represents a critical security flaw in the Sabberworm PHP CSS Parser library, which affects versions prior to 8.3.1. This issue stems from the improper handling of user-supplied input within the library's parsing functions, specifically the allSelectors() and getSelectorsBySpecificity() methods. The vulnerability is classified under CWE-94, which denotes "Improper Control of Generation of Code ('Code Injection')" and aligns with the ATT&CK technique T1059.007 for "Command and Scripting Interpreter: PowerShell" and T1133 for "External Remote Services" as it enables remote code execution through manipulated CSS input. The library's failure to properly sanitize or validate input data before processing creates a dangerous attack vector where malicious actors can inject arbitrary code into the application's execution context.
The technical exploitation of this vulnerability occurs when an attacker provides crafted CSS input that triggers the eval() function within the parser's internal processing logic. When either allSelectors() or getSelectorsBySpecificity() methods receive untrusted input, the parser executes the eval() function on attacker-controlled data, allowing for arbitrary code execution on the server where the vulnerable library is implemented. This behavior constitutes a direct code injection vulnerability where the application's trust boundary is violated, enabling attackers to execute malicious commands with the privileges of the web application. The flaw is particularly dangerous because it can be triggered through any input that eventually reaches these specific parser methods, making it accessible through various attack vectors including file uploads, form submissions, or API endpoints that process CSS content.
The operational impact of CVE-2020-13756 extends beyond simple remote code execution, as it can lead to complete system compromise and data breaches. Attackers can leverage this vulnerability to establish persistent backdoors, exfiltrate sensitive data, escalate privileges, or deploy additional malicious payloads. The vulnerability affects web applications that utilize the Sabberworm PHP CSS Parser for processing user-generated CSS content, which is common in content management systems, web applications with rich text editors, and any platform that allows users to upload or submit CSS files. Organizations using vulnerable versions of this library face significant risk of unauthorized access, data loss, and potential regulatory compliance violations, especially in environments where sensitive information is processed or stored.
Mitigation strategies for CVE-2020-13756 primarily focus on immediate remediation through version updates to 8.3.1 or later, which addresses the improper eval() usage in the affected methods. Security teams should conduct comprehensive inventory assessments to identify all systems utilizing vulnerable versions of the Sabberworm PHP CSS Parser, particularly in environments with multiple applications or dependencies that may indirectly reference the library. Additionally, implementing proper input validation and sanitization measures at application boundaries can provide defense-in-depth protection, though this is secondary to the primary remediation approach. Organizations should also consider implementing web application firewalls, monitoring for suspicious CSS parsing activities, and conducting regular security assessments to detect potential exploitation attempts. The vulnerability demonstrates the critical importance of validating and sanitizing all user inputs, especially in libraries that process structured data formats, and highlights the necessity of keeping third-party dependencies updated to prevent exploitation of known vulnerabilities.