CVE-2019-18798 in LibSass
Summary
by MITRE
LibSass before 3.6.3 allows a heap-based buffer over-read in Sass::weaveParents in ast_sel_weave.cpp.
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Analysis
by VulDB Data Team • 02/05/2024
The vulnerability identified as CVE-2019-18798 represents a critical heap-based buffer over-read flaw in LibSass versions prior to 3.6.3. This issue specifically affects the Sass::weaveParents function within the ast_sel_weave.cpp file, which is responsible for processing CSS selectors during the compilation process. The vulnerability arises from insufficient bounds checking when handling memory allocations for selector weaving operations, creating a scenario where an attacker can trigger memory access beyond allocated buffer boundaries. This type of vulnerability falls under the category of memory safety issues and is classified as CWE-125 according to the Common Weakness Enumeration catalog, which tracks insecure coding practices related to buffer overflows and memory access violations.
The technical implementation of this vulnerability occurs during the processing of complex CSS selector structures where LibSass attempts to weave parent selectors together. When the Sass::weaveParents function processes certain input patterns, it fails to properly validate the size of data structures before accessing memory locations, leading to a situation where the program reads from memory locations that may contain sensitive data or cause program instability. The heap-based nature of the vulnerability indicates that the over-read occurs in dynamically allocated memory regions, making it particularly challenging to predict and exploit consistently. This flaw can be triggered through carefully crafted CSS input that contains nested or complex selector combinations, potentially allowing attackers to read arbitrary memory contents from the heap.
The operational impact of CVE-2019-18798 extends beyond simple memory corruption, as it can potentially lead to information disclosure, denial of service, or in more severe cases, remote code execution depending on the execution environment. When exploited, this vulnerability can expose sensitive information stored in adjacent memory locations, including but not limited to cryptographic keys, session tokens, or other confidential data that may be present in the heap. The vulnerability is particularly concerning in web applications that use LibSass for CSS compilation, as it can be triggered through user-supplied CSS content, making it a potential vector for server-side exploitation. The attack surface is broadened by the widespread use of LibSass in various web frameworks and static site generators, including popular tools like node-sass and various Ruby-based web applications. This vulnerability aligns with ATT&CK technique T1203 which involves the exploitation of memory corruption vulnerabilities to gain unauthorized access to system resources.
Mitigation strategies for CVE-2019-18798 primarily involve upgrading to LibSass version 3.6.3 or later, which contains the necessary patches to address the buffer over-read condition. Organizations should prioritize updating their dependency management systems to ensure all instances of LibSass are patched, particularly in production environments where user input is processed through CSS compilation pipelines. Additionally, implementing input validation and sanitization measures can provide defense-in-depth protection, though this is secondary to the core patching requirement. Security monitoring should include detection of unusual memory access patterns and potential exploitation attempts through malformed CSS input. The vulnerability demonstrates the importance of maintaining up-to-date dependencies and implementing robust security testing practices including fuzzing and memory safety analysis to identify similar issues before they can be exploited in the wild.