CVE-2018-19218 in LibSass
Summary
by MITRE
In LibSass 3.5-stable, there is an illegal address access at Sass::Parser::parse_css_variable_value_token that will lead to a DoS attack.
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Analysis
by VulDB Data Team • 09/09/2025
The vulnerability identified as CVE-2018-19218 affects LibSass version 3.5-stable and represents a critical memory access violation that can be exploited to cause denial of service conditions. This issue manifests within the Sass::Parser::parse_css_variable_value_token function where improper input validation leads to illegal memory access patterns. The flaw exists in the parsing logic responsible for processing CSS variable values within the LibSass library, which is commonly used as a Sass compiler implementation in various web development frameworks and tools. When maliciously crafted input is processed through this parser, it triggers an access violation that can result in application crashes or complete system unavailability.
The technical implementation of this vulnerability stems from insufficient boundary checking and input sanitization within the CSS variable parsing routine. The Sass::Parser::parse_css_variable_value_token function fails to properly validate the structure and content of CSS variable tokens before attempting to access memory locations. This type of vulnerability falls under CWE-125: Out-of-bounds Read, which occurs when a program attempts to read memory beyond the bounds of a buffer or allocated memory region. The flaw allows attackers to craft specific input sequences that cause the parser to access invalid memory addresses, leading to segmentation faults or similar memory access violations. The vulnerability is particularly concerning because it can be triggered through normal parsing operations when processing user-supplied CSS content or Sass files that contain malformed variable declarations.
The operational impact of CVE-2018-19218 extends beyond simple application crashes to potentially enable broader system compromise when the affected library is used in web applications or development environments. Since LibSass is widely integrated into popular web frameworks, build tools, and static site generators, exploitation of this vulnerability could affect numerous applications and services. The denial of service condition can be particularly damaging in production environments where continuous availability is critical, as it can render web applications or development tools unusable until the underlying parser is restarted or patched. Additionally, this vulnerability may serve as a stepping stone for more sophisticated attacks, as memory corruption issues often provide opportunities for further exploitation techniques that align with ATT&CK technique T1059.001: Command and Scripting Interpreter for executing malicious code through compromised parsing components.
Mitigation strategies for this vulnerability should focus on immediate patching of affected LibSass versions, as the issue was resolved in subsequent releases through proper input validation and memory boundary checks. Organizations should implement comprehensive input sanitization measures when processing CSS or Sass content, particularly in applications that accept user-generated stylesheets or allow dynamic compilation of user-supplied content. The fix typically involves strengthening the parser's validation logic to ensure that CSS variable tokens are properly structured before memory access operations are performed. Security teams should also consider implementing runtime monitoring and intrusion detection systems that can identify anomalous parsing behavior indicative of exploitation attempts. Regular security assessments of web applications and development environments should include checks for vulnerable LibSass versions, and automated dependency scanning tools should be employed to identify and remediate affected components throughout the software supply chain.