CVE-2026-86071 in Junrar
Summary
by MITRE • 09/16/2026
Junrar is an open source Java RAR archive library. Prior to version 7.6.1, LocalFolderExtractor in src/main/java/com/github/junrar/LocalFolderExtractor.java can create directories outside the intended extraction root when processing a crafted archive entry. LocalFolderExtractor.createFile() validates only the final canonical file path, while LocalFolderExtractor.makeFile() creates intermediate path segments with unchecked mkdir() calls. An entry can therefore make the final path resolve inside the destination while causing intermediate directory creation outside it, enabling filesystem pollution or file-versus-directory squatting that can make later security-sensitive writes fail. The demonstrated impact is directory creation, not unconditional arbitrary file-content write. This issue is fixed in version 7.6.1.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified within the Junrar library, specifically affecting versions prior to 7.6.1, represents a critical path traversal and improper input validation flaw located in the LocalFolderExtractor component. As an open-source Java implementation for handling RAR archive formats, Junrar is frequently utilized by applications requiring robust compression utilities. The core technical deficiency lies within the file extraction logic, specifically involving the interaction between the createFile() and makeFile() methods. When processing a crafted archive entry containing maliciously constructed path segments, the library fails to enforce strict containment of intermediate directory creation operations relative to the designated extraction root. This architectural oversight allows an attacker to manipulate the filesystem structure in ways that were not anticipated by the original design, leading to significant operational risks for any application relying on this library for untrusted archive processing.
The technical mechanism of exploitation hinges on a discrepancy between how final paths are validated versus how intermediate directories are created. The LocalFolderExtractor.createFile() method performs validation only on the canonical path of the target file, ensuring that the ultimate destination resides within the intended extraction directory. However, the preceding step, handled by LocalFolderExtractor.makeFile(), utilizes unchecked mkdir() calls to create necessary parent directories along the path. Because these intermediate creation steps do not apply the same rigorous boundary checks as the final file validation, an attacker can craft a RAR entry with nested paths that resolve correctly at the leaf level but require directory structures outside the allowed root during their construction phase. This allows the extraction process to inadvertently create folders in arbitrary locations on the host filesystem, effectively bypassing the sandbox or chroot-like restrictions intended by the application using Junrar.
The operational impact of this vulnerability extends beyond simple unauthorized file creation. While it does not permit direct arbitrary write access to sensitive system files due to the final path validation, it enables severe filesystem pollution and directory squatting attacks. By creating unexpected directories in critical locations, an attacker can interfere with subsequent security-sensitive operations performed by the host application or other processes. For instance, if a maliciously created directory occupies a path that a legitimate process expects to be empty or writable for configuration files or temporary data, it can cause those writes to fail, leading to denial of service conditions or unexpected application behavior. Furthermore, this technique facilitates file-versus-directory squatting, where the presence of an attacker-controlled directory blocks access to intended resources, potentially disrupting business logic or causing stability issues in dependent systems that rely on predictable filesystem states.
From a classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, as it involves path traversal leading to unauthorized resource access outside the restricted area. Additionally, it relates to CWE-732: Incorrect Permission Assignment for Critical Resource, given that the creation of directories in unintended locations can alter security permissions and ownership structures unexpectedly. In terms of adversary tactics, this behavior is consistent with ATT&CK technique T1564.001: Hidden Files and Directories (Hidden File), as attackers may use such techniques to obscure malicious artifacts or disrupt system operations by polluting the filesystem namespace without necessarily dropping executable payloads directly into protected areas.
To mitigate this vulnerability, organizations must immediately upgrade Junrar to version 7.6.1 or later, where the developers have addressed these path validation gaps by ensuring that all intermediate directory creation steps are subject to the same strict boundary checks as final file operations. For applications unable to update immediately due to dependency constraints, implementing a custom wrapper around the extraction logic is recommended. This wrapper should manually validate every segment of the extracted path against the allowed root directory before passing it to the library methods. Additionally, deploying runtime application self-protection (RASP) solutions or utilizing operating system-level file integrity monitoring can help detect anomalous directory creation patterns that deviate from expected behavior during archive extraction processes. Regular security audits of third-party libraries and maintaining an up-to-date software bill of materials are essential practices to prevent similar path traversal issues in the future.