CVE-2017-11554 in LibSass
Summary
by MITRE
There is a stack consumption vulnerability in the lex function in parser.hpp (as used in sassc) in LibSass 3.4.5. A crafted input will lead to a remote denial of service.
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Analysis
by VulDB Data Team • 12/13/2022
The vulnerability identified as CVE-2017-11554 represents a critical stack consumption issue within the LibSass library version 3.4.5, specifically affecting the lex function in parser.hpp as utilized by sassc. This flaw manifests as a stack-based buffer overflow condition that can be exploited through carefully crafted input sequences. The vulnerability stems from insufficient input validation and improper stack management during the lexical analysis phase of sass document processing, creating a scenario where recursive parsing operations can exhaust available stack memory resources. The affected component operates within the broader context of CSS preprocessor functionality, where LibSass serves as the core engine for compiling sass/scss markup into standard css output. This vulnerability directly impacts the stability and availability of applications relying on sassc for stylesheet compilation, as the stack exhaustion leads to immediate process termination and system instability.
The technical implementation of this vulnerability exploits the recursive nature of the lex function within the parser.hpp file, which processes input tokens during sass document parsing. When confronted with maliciously constructed input sequences, the lex function can trigger excessive stack frame allocations through recursive calls that do not properly terminate or bound their execution depth. The flaw operates at the level of lexical analysis where the parser attempts to identify and categorize input tokens, but fails to enforce reasonable limits on recursive parsing depth. This behavior aligns with CWE-772, which addresses insufficient resource management, and specifically manifests as a stack-based buffer overflow that consumes excessive memory resources. The vulnerability is particularly concerning because it allows remote attackers to induce denial of service conditions without requiring any privileged access or authentication, making it highly exploitable in networked environments.
The operational impact of CVE-2017-11554 extends beyond simple service disruption to encompass broader system reliability concerns for applications utilizing LibSass. When exploited, this vulnerability can cause complete application crashes, process termination, and denial of service conditions that affect legitimate users attempting to compile sass documents. The vulnerability affects any system where sassc is used as a command-line interface for compiling sass/scss files, including web development environments, build systems, and automated deployment pipelines. In enterprise contexts, this vulnerability can compromise continuous integration/continuous deployment workflows, potentially blocking software releases and development cycles. The remote exploitation capability means that attackers can target systems without physical access, making it particularly dangerous for web applications that process user-uploaded sass content or for systems that accept external sass inputs. From an attack framework perspective, this vulnerability maps to ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a classic example of resource exhaustion attacks that can be executed with minimal privileges.
Mitigation strategies for CVE-2017-11554 should focus on immediate version upgrades to LibSass 3.5.0 or later, which contain the necessary patches to address the stack consumption issue. Organizations should implement comprehensive input validation and sanitization measures for any sass content processed by systems using affected LibSass versions. System administrators should consider implementing resource limits and process monitoring to detect and prevent stack exhaustion conditions before they cause complete system failures. Additionally, deploying intrusion detection systems that can identify suspicious input patterns targeting lexical analysis functions may provide early warning capabilities. The fix implemented in subsequent versions addresses the root cause by introducing proper recursion depth limits and stack management controls within the lex function, preventing the unbounded stack consumption that previously enabled denial of service attacks. Security teams should also conduct thorough vulnerability assessments to identify any other systems or applications that may be indirectly affected through dependencies on the vulnerable LibSass library.