CVE-2017-11556 in LibSass
Summary
by MITRE
There is a stack consumption vulnerability in the Parser::advanceToNextToken function in parser.cpp in LibSass 3.4.5. A crafted input may lead to remote denial of service.
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Analysis
by VulDB Data Team • 10/31/2019
The vulnerability identified as CVE-2017-11556 represents a critical stack consumption issue within the LibSass library version 3.4.5, specifically affecting the Parser::advanceToNextToken function in parser.cpp. This flaw manifests as a denial of service condition that can be triggered remotely through carefully crafted input sequences. The vulnerability stems from improper handling of stack memory allocation during the parsing process, where the parser fails to adequately validate or limit recursive stack usage when processing malformed input structures. Such vulnerabilities are particularly dangerous in parsing libraries that are widely used across web applications and build systems, as they can be exploited to crash services or consume excessive system resources.
The technical implementation of this vulnerability resides in the parser's recursive descent parsing algorithm, which relies heavily on stack-based memory management for tracking parsing state and token transitions. When the Parser::advanceToNextToken function encounters malformed or specially crafted input, it can trigger excessive stack consumption through recursive calls or deep nesting of parsing operations. This behavior aligns with CWE-401, which addresses improper handling of stack-based memory consumption, and specifically relates to the CWE-121 category of stack-based buffer overflow conditions. The vulnerability operates at the intersection of parsing logic and memory management, where the parser's inability to properly bound stack usage creates a pathway for attackers to exhaust available stack space.
From an operational perspective, this vulnerability presents a significant risk to systems utilizing LibSass for CSS processing, particularly in web applications, static site generators, and build automation tools. Attackers can exploit this weakness by submitting malicious input to CSS processing endpoints, leading to service disruption through stack exhaustion or process termination. The remote exploitation capability means that systems processing user-supplied CSS content are particularly vulnerable, as the denial of service can occur without requiring local system access or authentication. This vulnerability impacts the availability aspect of the system's security triad, potentially allowing attackers to perform denial of service attacks against web applications, content management systems, and static site generators that rely on LibSass for stylesheet compilation.
The mitigation strategy for CVE-2017-11556 involves immediate upgrading to LibSass version 3.5.0 or later, where the stack consumption issue has been addressed through improved input validation and stack usage monitoring. System administrators should also implement input sanitization measures for any user-supplied CSS content, including size limitations and structural validation before processing. Additionally, deploying intrusion detection systems that can identify patterns of stack-based memory consumption anomalies can help detect exploitation attempts. Organizations should also consider implementing rate limiting and resource quotas for CSS processing endpoints to prevent exploitation from consuming excessive system resources. The fix implemented in newer versions typically involves introducing proper stack depth limits and more robust recursive call handling within the parser's token advancement logic, aligning with ATT&CK technique T1499.004 for resource consumption attacks and ensuring proper memory management practices that prevent stack-based vulnerabilities.