CVE-2026-104086 in 7-Zip
Summary
by MITRE • 10/14/2026
7-Zip before 26.03 contains an uninitialized memory disclosure vulnerability in the SquashFS v4 archive handler (SquashfsHandler.cpp) that allows attackers to leak heap memory by crafting a malicious archive where the superblock’s no_ids field is set so that the final metadata block’s expected size is miscomputed and bypasses the size validation check. By listing or extracting the crafted archive, attackers can cause the application to return uninitialized heap memory as the kpidUserId and kpidGroupId properties, exposing potentially sensitive process memory contents to the user.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified in versions of 7-Zip prior to release 26.03 represents a critical information disclosure flaw located within the SquashFS v4 archive handler implementation, specifically in the file SquashfsHandler.cpp. This issue stems from improper initialization of memory structures during the parsing and processing of compressed archives that utilize the SquashFS filesystem format version four. The core technical deficiency involves the handling of metadata blocks associated with user and group identifiers. When an attacker crafts a malicious archive where the superblock’s no_ids field is manipulated, it triggers a logic error in the calculation of the final metadata block's expected size. This miscomputation allows the crafted input to bypass standard size validation checks that are designed to prevent buffer overflows or out-of-bounds reads. Consequently, when the application attempts to read these malformed blocks, it accesses memory regions that have not been properly initialized by the software logic.
From an operational perspective, this flaw enables remote attackers to leak sensitive heap memory contents through simple user interactions such as listing directory contents within the archive or extracting files from it. The specific data exposed includes uninitialized values for the kpidUserId and kpidGroupId properties. In many execution contexts, these fields may inadvertently contain remnants of previous operations stored in the same heap allocation. This can lead to the exposure of sensitive process memory contents, which might include cryptographic keys, session tokens, passwords, or other proprietary information previously held by the application before it was freed or reused for this archive parsing task. The impact is particularly severe because 7-Zip is widely used across enterprise and personal environments, often processing files from untrusted sources such as email attachments or downloaded web content.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, specifically manifesting as a memory disclosure due to uninitialized variables. It also maps to the MITRE ATT&CK framework under T1530: Data from Local System, where adversaries seek to access data stored on local systems through exploitation of software vulnerabilities rather than direct system compromise. The attack vector is classified as remote with low complexity, requiring only that a user opens or lists a maliciously crafted SquashFS archive.
To mitigate this risk, organizations and individual users must immediately upgrade 7-Zip to version 26.03 or later, where the developer has addressed the logic error in the metadata block size validation and ensured proper initialization of memory structures before use. Until an update is applied, administrators should enforce strict policies regarding the processing of archive files from untrusted sources, potentially utilizing sandboxed environments for extraction tasks to limit the blast radius if a vulnerability were exploited. Additionally, deploying endpoint detection and response solutions that monitor for anomalous memory access patterns or unexpected data exfiltration can provide secondary layers of defense against exploitation attempts targeting this specific flaw in legacy versions of the software.