CVE-2018-20822 in LibSass
Summary
by MITRE
LibSass 3.5.4 allows attackers to cause a denial-of-service (uncontrolled recursion in Sass::Complex_Selector::perform in ast.hpp and Sass::Inspect::operator in inspect.cpp).
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Analysis
by VulDB Data Team • 09/06/2023
The vulnerability identified as CVE-2018-20822 affects LibSass version 3.5.4, a widely used C++ library for compiling Sass stylesheet syntax into CSS. This issue manifests as an uncontrolled recursion flaw within the Sass::Complex_Selector::perform function located in ast.hpp and the Sass::Inspect::operator function in inspect.cpp. The flaw represents a classic denial-of-service vulnerability that can be exploited by malicious actors to disrupt service availability. The recursive nature of the flaw means that an attacker can craft specific input that causes the library to enter infinite recursive loops, consuming system resources and ultimately leading to application crashes or system unresponsiveness. This vulnerability is particularly concerning given LibSass's extensive usage in web development environments and content management systems where Sass compilation is a common operation. The flaw falls under CWE-674, which specifically addresses uncontrolled recursion in software systems, and aligns with ATT&CK technique T1499.100, which covers network denial of service attacks through resource exhaustion. The vulnerability can be triggered during the parsing and compilation of Sass files, making it applicable to any application that utilizes LibSass for stylesheet processing. Attackers can exploit this by submitting carefully crafted Sass code that contains nested selectors or complex nesting structures that will cause the recursive functions to execute indefinitely.
The operational impact of this vulnerability extends beyond simple service disruption, as it can affect entire web applications or development environments that depend on LibSass for CSS generation. When exploited, the uncontrolled recursion consumes CPU cycles and memory resources rapidly, potentially leading to system instability or complete service outages. This is particularly problematic in production environments where continuous availability is critical, as a single malicious input can compromise the stability of the entire system. The vulnerability affects both development and deployment workflows since LibSass is commonly used in build processes, continuous integration pipelines, and live web applications. The flaw's exploitation requires minimal technical expertise, making it accessible to a broad range of threat actors from script kiddies to sophisticated attackers. Systems that process user-generated content or allow arbitrary Sass input are especially vulnerable, as attackers can inject malicious code that triggers the recursive behavior during normal processing operations. The memory consumption patterns associated with this vulnerability can also lead to memory exhaustion attacks that may affect system performance or cause applications to crash entirely.
Mitigation strategies for CVE-2018-20822 should focus on immediate patching of affected LibSass versions, as the vulnerability has been addressed in subsequent releases. Organizations should implement input validation and sanitization measures to filter potentially malicious Sass code before processing, particularly in environments where user inputs are accepted. The implementation of resource limits and timeout mechanisms can help prevent exploitation by restricting the amount of CPU and memory that individual processing operations can consume. Additionally, monitoring systems should be deployed to detect unusual resource consumption patterns that may indicate exploitation attempts. Security teams should conduct comprehensive vulnerability assessments to identify all systems utilizing LibSass and ensure proper patch management protocols are in place. Network segmentation and access controls can help limit the potential impact of successful exploitation by restricting access to affected systems. The vulnerability also underscores the importance of secure coding practices and thorough testing of recursive functions within software libraries. Organizations should consider implementing automated security scanning tools that can detect and prevent the execution of potentially malicious Sass code. Regular security updates and penetration testing should be conducted to identify similar vulnerabilities in other components of the software stack. The remediation process should include not only patching the library but also reviewing and strengthening the overall security posture of systems that rely on Sass compilation for web development workflows.