CVE-2018-19797 in LibSass
Summary
by MITRE
In LibSass 3.5.5, a NULL Pointer Dereference in the function Sass::Selector_List::populate_extends in SharedPtr.hpp (used by ast.cpp and ast_selectors.cpp) may cause a Denial of Service (application crash) via a crafted sass input file.
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Analysis
by VulDB Data Team • 06/12/2023
The vulnerability CVE-2018-19797 represents a critical null pointer dereference flaw within LibSass version 3.5.5 that specifically affects the Sass stylesheet preprocessor library. This issue manifests in the Sass::Selector_List::populate_extends function located within the SharedPtr.hpp header file, which serves as a core component in the library's AST (Abstract Syntax Tree) processing pipeline. The vulnerability arises when the library processes maliciously crafted sass input files that contain malformed selector structures, leading to a scenario where a null pointer is dereferenced during the extension population phase of the parsing process.
The technical implementation of this vulnerability stems from insufficient input validation and error handling within the selector list processing logic. When LibSass encounters a specially constructed sass file containing malformed or unexpected selector patterns, the populate_extends function fails to properly validate pointer references before attempting to access them. This particular flaw exists in the shared pointer management system that handles memory allocation and deallocation for AST nodes, making it particularly dangerous as it operates at a fundamental level of the library's parsing infrastructure. The issue affects both ast.cpp and ast_selectors.cpp files, indicating that the vulnerability spans multiple components of the library's selector processing pipeline.
From an operational perspective, this vulnerability enables a remote attacker to execute a denial of service attack against applications that utilize LibSass for stylesheet compilation. The attack requires only the ability to submit crafted sass input files to the target system, making it relatively easy to exploit in environments where user input is processed through LibSass. When successfully exploited, the vulnerability causes the application to crash immediately upon processing the malicious input, resulting in complete service disruption. This type of vulnerability is particularly concerning in web applications, content management systems, or any platform that accepts user-generated sass content for compilation and rendering.
The impact of this vulnerability aligns with CWE-476, which identifies null pointer dereference as a common weakness in software security, and maps to ATT&CK technique T1499.004 for network denial of service attacks. Organizations using LibSass in production environments face significant risk as this vulnerability can be leveraged to cause service outages without requiring any advanced privileges or specialized knowledge. The attack vector is particularly dangerous because it can be triggered through simple file uploads or content submission mechanisms, making it an attractive target for automated exploitation. Security practitioners should note that this vulnerability demonstrates the importance of robust input validation and proper error handling in parsing libraries, as these components often serve as critical attack surfaces in modern web applications.
Mitigation strategies for CVE-2018-19797 require immediate patching of affected LibSass installations to version 3.5.6 or later, which contains the necessary fixes to properly validate pointer references during selector list processing. Organizations should also implement input validation measures that filter or sanitize sass content before processing, particularly in applications that accept user-generated content. Additionally, deploying network-based intrusion detection systems that can identify and block malicious sass input patterns may provide an additional layer of defense. The vulnerability highlights the importance of maintaining up-to-date dependencies and implementing comprehensive security testing procedures that include fuzzing and static analysis of parsing components to identify similar weaknesses before they can be exploited in the wild.