CVE-2026-119602 in libmspack
Summary
by MITRE • 10/30/2026
libmspack through 0.11alpha contains an uninitialized memory disclosure vulnerability in the MS-ZIP decompressor that allows attackers to leak heap data via unchecked back-reference distances. Attackers can supply a crafted MS-ZIP CAB file whose first block begins with a back-reference, causing cabd->extract() to write up to 32 KiB of stale heap memory into extracted output.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified in libmspack versions through 0.11alpha represents a critical information disclosure flaw within the MS-ZIP decompression module. This library is widely utilized for handling Microsoft Cabinet files, which are common archives used by Windows operating systems and various software distribution mechanisms. The specific defect resides in the logic governing back-reference distances during the decompression process. In LZ77-based compression algorithms like MS-ZIP, data recovery relies on copying sequences from previously decoded output rather than reading literal bytes. When a compressed stream contains a back-reference instruction, the decoder must calculate how many bytes to copy from an earlier position in the already-decompressed buffer. The flaw arises because libmspack fails to properly initialize or validate certain memory states before performing these copy operations under specific edge cases involving crafted input files.
The technical root cause of this vulnerability is an uninitialized memory disclosure triggered by a malformed MS-ZIP CAB file. Specifically, when an attacker constructs a CAB archive where the first block begins with a back-reference instruction rather than literal data, the decompression engine attempts to resolve the reference distance without ensuring that the target buffer region has been properly initialized or bounded correctly relative to the current output position. Because this occurs at the very start of extraction, there is no prior valid decoded data in the immediate vicinity to serve as safe source material for the copy operation. Consequently, instead of copying from a known-good portion of the decompressed stream, the function writes up to 32 KiB of stale heap memory into the extracted output buffer. This behavior effectively leaks whatever random data previously occupied that region of the application's heap space, which may include sensitive information such as cryptographic keys, session tokens, or other proprietary data from previous operations within the same process context.
From an operational perspective, this vulnerability poses a significant risk to applications that rely on libmspack for processing untrusted archive files. An attacker can exploit this flaw by distributing maliciously crafted CAB files through email attachments, web downloads, or network services. Upon extraction, the application inadvertently exposes internal memory contents to the output stream, which is then written to disk or transmitted over a network connection depending on how the consuming application handles the decompressed data. This constitutes an arbitrary read of heap memory and can lead to severe confidentiality breaches. The impact extends beyond simple data leakage; if the leaked memory contains pointers or control structures, it could potentially aid in further exploitation attempts such as heap spraying or information gathering for subsequent attacks against the host system.
This vulnerability aligns with CWE-200, which categorizes exposure of sensitive information to an unauthorized actor, and specifically relates to improper initialization leading to information leakage. In terms of attack vectors, this falls under ATT&CK technique T1537, where attackers utilize specific file formats or protocols to exfiltrate data from a compromised system. The exploitation requires the attacker to craft a precise binary payload that triggers the uninitialized read condition at the start of the decompression stream, making it somewhat complex but highly effective for targeted attacks against systems processing archives.
Mitigation strategies primarily involve updating libmspack to version 0.12 or later where this issue has been addressed by developers who implemented stricter bounds checking and proper memory initialization routines within the MS-ZIP decoder. For organizations unable to immediately update their dependencies, defensive coding practices should be enforced in any wrapper code that consumes libmspack outputs. This includes validating the integrity of decompressed data before further processing or transmission. Additionally, deploying intrusion detection systems capable of identifying anomalous file structures associated with archive exploitation can provide an additional layer of defense against attempts to trigger this vulnerability. Regular security audits and static analysis tools configured to detect uninitialized variable usage in C/C++ codebases are also recommended to prevent similar flaws from being introduced into other components of the software ecosystem.