CVE-2017-12963 in LibSass
Summary
by MITRE
There is an illegal address access in Sass::Eval::operator() in eval.cpp of LibSass 3.4.5, leading to a remote denial of service attack. NOTE: this is similar to CVE-2017-11555 but remains exploitable after the vendor's CVE-2017-11555 fix (available from GitHub after 2017-07-24).
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Analysis
by VulDB Data Team • 11/09/2019
The vulnerability identified as CVE-2017-12963 represents a critical remote denial of service flaw within the LibSass library version 3.4.5. This issue manifests in the Sass::Eval::operator() function located within the eval.cpp source file, where improper memory management leads to illegal address access patterns. The flaw specifically affects the evaluation engine of LibSass, which is widely used for processing sass and scss stylesheet files in web development environments. The vulnerability's impact extends beyond simple functionality degradation as it enables attackers to craft malicious input that triggers memory access violations, thereby causing the application to crash or become unresponsive.
The technical exploitation of this vulnerability occurs through carefully constructed sass/scss code that, when processed by the affected LibSass library, causes the evaluation engine to attempt accessing memory locations that are either unmapped or unauthorized for the current execution context. This illegal address access pattern typically results in segmentation faults or access violations that terminate the processing thread or entire application instance. The vulnerability's persistence despite the fix for CVE-2017-11555 indicates that the underlying memory management issue was not fully addressed in the vendor's patch, leaving systems vulnerable to repeated exploitation. This particular flaw aligns with CWE-125, which describes out-of-bounds read conditions, and represents a classic example of improper memory access handling in interpreted languages or compilation engines.
From an operational perspective, this vulnerability poses significant risks to web applications and development environments that rely on LibSass for stylesheet processing. Attackers can leverage this flaw by submitting maliciously crafted sass/scss files to applications that process user input through LibSass, potentially causing widespread service disruption across multiple systems. The remote nature of the attack means that vulnerable systems can be compromised from external networks without requiring local access or authentication. This vulnerability particularly affects content management systems, web development frameworks, and any application that accepts and processes user-generated sass content, making it a critical concern for organizations maintaining web-facing applications. The persistence of the vulnerability even after the previous CVE-2017-11555 patch demonstrates the complexity of memory management issues in compilation engines and highlights the importance of thorough vulnerability assessment.
Organizations should implement immediate mitigations including updating to LibSass versions that contain proper fixes for this vulnerability, typically those released after July 24, 2017, when the vendor's patch became available. System administrators should also consider implementing input validation and sanitization measures for any user-supplied sass content, particularly in applications that process external stylesheet files. Additionally, organizations may need to deploy network-based intrusion detection systems that can identify and block malicious sass content patterns. The vulnerability's classification under ATT&CK technique T1499.004, which covers network denial of service, indicates that defensive measures should include monitoring for unusual resource consumption patterns and implementing rate limiting for stylesheet processing operations. Regular security assessments and dependency updates remain crucial for maintaining protection against similar memory access vulnerabilities in software libraries.