CVE-2018-14679 in libmspack
Summary
by MITRE
An issue was discovered in mspack/chmd.c in libmspack before 0.7alpha. There is an off-by-one error in the CHM PMGI/PMGL chunk number validity checks, which could lead to denial of service (uninitialized data dereference and application crash).
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Analysis
by VulDB Data Team • 04/27/2023
The vulnerability identified as CVE-2018-14679 represents a critical denial of service flaw within the libmspack library, specifically in the mspack/chmd.c component. This issue affects versions prior to 0.7alpha and stems from an off-by-one error in the validation logic for CHM PMGI/PMGL chunk numbers. The flaw occurs during the processing of Microsoft Compiled HTML Help files, which are commonly used for documentation and help systems in Windows environments. The vulnerability is particularly concerning as it can be triggered through malformed input files that exploit the boundary condition in the chunk number validation mechanism.
The technical implementation of this vulnerability involves an uninitialized data dereference that occurs when the application attempts to access memory locations that have not been properly initialized. This type of error typically arises when programmers fail to validate input parameters or when boundary conditions are not properly handled in array indexing operations. In this case, the off-by-one error specifically affects the validation checks for PMGI (Primary MetaData Index) and PMGL (Primary MetaData Link) chunks within CHM files, where the boundary condition check fails to properly validate the maximum allowable chunk numbers. The flaw can be categorized under CWE-129 as "Improper Validation of Array Index" and also relates to CWE-476 as "NULL Pointer Dereference" when uninitialized memory is accessed.
The operational impact of this vulnerability extends beyond simple denial of service, as it can be exploited by attackers to cause application crashes and potentially disrupt legitimate operations. When a vulnerable application processes a specially crafted CHM file, the uninitialized data dereference leads to unpredictable behavior and system instability. This vulnerability is particularly dangerous in environments where applications automatically process user-supplied CHM files, such as web applications, document viewers, or help system integrations. The flaw can be exploited through various attack vectors including email attachments, web downloads, or file sharing platforms, making it a significant concern for organizations that rely on CHM file processing capabilities.
Mitigation strategies for CVE-2018-14679 should focus on immediate version updates to libmspack 0.7alpha or later, which contains the necessary patches to address the off-by-one error in chunk number validation. Organizations should also implement input validation measures that can detect and reject malformed CHM files before they reach the vulnerable processing code. Additionally, deployment of application sandboxing techniques and privilege separation can help limit the impact of successful exploitation attempts. From a defensive perspective, this vulnerability aligns with ATT&CK technique T1203 as "Exploitation for Client Execution" and demonstrates the importance of maintaining up-to-date libraries and components. Network monitoring should be enhanced to detect potential exploitation attempts through malformed CHM file delivery, and security teams should implement regular vulnerability assessments to identify other potential boundary condition flaws in similar libraries and applications. The fix implemented in the patched version addresses the core validation logic by properly handling the boundary conditions and ensuring that all memory accesses are properly initialized before use.