CVE-2019-18797 in LibSass
Summary
by MITRE
LibSass 3.6.1 has uncontrolled recursion in Sass::Eval::operator()(Sass::Binary_Expression*) in eval.cpp.
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Analysis
by VulDB Data Team • 02/05/2024
The vulnerability identified as CVE-2019-18797 represents a critical security flaw in LibSass version 3.6.1, a widely used C++ library for compiling Sass stylesheet language into CSS. This issue manifests as uncontrolled recursion within the Sass::Eval::operator() function located in the eval.cpp source file, specifically when processing Sass::Binary_Expression objects. The flaw occurs during the evaluation phase of Sass compilation, where the library fails to properly validate or limit recursive operations that can be triggered through maliciously crafted Sass code. This vulnerability falls under the CWE-674 category of "Uncontrolled Recursion" which is classified as a weakness that can lead to denial of service conditions and potentially more severe consequences depending on the execution environment.
The technical implementation of this vulnerability exploits the recursive nature of Sass expression evaluation without proper safeguards against infinite recursion loops. When a malicious user crafts a Sass file containing carefully constructed binary expressions that trigger recursive evaluation patterns, the LibSass library enters an unbounded recursion cycle. This occurs because the evaluation function does not implement adequate depth checking or recursion limits to prevent the call stack from growing indefinitely. The recursive nature of the flaw means that each recursive call consumes stack space, and without proper bounds, the system can quickly exhaust available memory resources or cause stack overflow conditions. This type of vulnerability is particularly dangerous in web applications where user input is processed through Sass compilation, as it can be exploited to cause denial of service attacks against web servers or applications that rely on LibSass for dynamic stylesheet generation.
The operational impact of CVE-2019-18797 extends beyond simple denial of service scenarios, as it can be leveraged in various attack vectors within software development and deployment environments. Applications that accept user-generated Sass content or process external Sass files through LibSass 3.6.1 are particularly vulnerable to exploitation. This includes web applications, content management systems, static site generators, and development tools that utilize LibSass for stylesheet compilation. The vulnerability can be exploited by attackers who craft malicious Sass files designed to trigger the recursive evaluation patterns, potentially causing system crashes, resource exhaustion, or even allowing for more sophisticated attacks depending on the hosting environment. In continuous integration pipelines or automated build systems that process Sass files, this vulnerability could be weaponized to disrupt development workflows or cause cascading failures across multiple dependent services.
Mitigation strategies for CVE-2019-18797 should focus on immediate version upgrades to LibSass 3.6.2 or later, which contains the necessary fixes to prevent uncontrolled recursion in the Sass evaluation process. Organizations should implement comprehensive patch management procedures to ensure all systems utilizing LibSass are updated promptly. Additionally, input validation and sanitization should be implemented at the application level to prevent untrusted Sass content from being processed through the vulnerable library. Security teams should consider implementing runtime monitoring to detect unusual memory consumption patterns or stack overflow conditions that may indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under the T1499.004 technique for "Network Denial of Service" and potentially T1059.001 for "Command and Scripting Interpreter" when considering the broader exploitation context. Organizations should also consider implementing sandboxing mechanisms for Sass compilation processes to isolate potential recursive evaluation attacks and prevent them from affecting core system resources. The fix implemented in subsequent versions typically involves adding recursion depth limits and proper stack management to prevent the uncontrolled growth of recursive calls during Sass expression evaluation.