CVE-2026-39245
Summary
by MITRE • 07/10/2026
decompress before 4.2.2 contains an improper path containment check that enables directory traversal and arbitrary file write. The safeMakeDir function (index.js line 29) and the extraction path validation (index.js line 106) use String.indexOf() to verify the resolved path is within the output directory: realDestinationDir.indexOf(realOutputPath) !== 0. This check is flawed because it does not enforce a path separator boundary. For example, "/tmp/app_config".indexOf("/tmp/app") returns 0, incorrectly passing the check even though /tmp/app_config is outside /tmp/app. Combined with the unvalidated symlink creation in the same package, an attacker can write arbitrary files to directories adjacent to the extraction target. This is a bypass of the fix for CVE-2020-12265. The correct check requires appending a path separator: realParentPath.indexOf(realOutputPath + path.sep) !== 0.
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Analysis
by VulDB Data Team • 07/10/2026
The decompress library versions prior to 4.2.2 contain a critical directory traversal vulnerability that stems from an improper path containment check implementation. This vulnerability allows attackers to bypass intended security restrictions and write files to arbitrary locations on the system. The flaw specifically manifests in the safeMakeDir function located at index.js line 29 and the extraction path validation logic at line 106, where developers employed String.indexOf() method to validate whether a resolved path remains within the designated output directory boundaries.
The technical implementation of this vulnerability relies on a fundamental flaw in string comparison logic that fails to enforce proper path separator boundaries. The current validation mechanism uses realDestinationDir.indexOf(realOutputPath) !== 0 which incorrectly passes validation when one path is a prefix of another without proper boundary checking. This approach creates a dangerous scenario where an attacker can craft malicious paths that appear to be contained within the target directory but actually extend beyond it. For instance, if the output directory is set to "/tmp/app" and an attacker attempts to write to "/tmp/app_config", the indexOf check returns 0 because "/tmp/app_config" begins with "/tmp/app", thereby incorrectly passing the validation despite the path being outside the intended directory scope.
This vulnerability represents a bypass of the previously addressed CVE-2020-12265 fix, indicating that the remediation was incomplete or improperly implemented. The flaw becomes particularly dangerous when combined with unvalidated symlink creation capabilities within the same package, creating a complete attack vector for arbitrary file write operations. Attackers can leverage this vulnerability to write files to directories adjacent to the intended extraction target, potentially leading to privilege escalation, data corruption, or system compromise. The impact extends beyond simple directory traversal as it enables attackers to place malicious files in critical system locations where they may be executed with elevated privileges.
The correct implementation should append a path separator to the validation check, specifically using realParentPath.indexOf(realOutputPath + path.sep) !== 0 which properly enforces boundary conditions between paths. This approach ensures that path components are properly separated and prevents the prefix matching issue that currently allows traversal attacks. Organizations should immediately update to decompress version 4.2.2 or later to address this vulnerability, as it falls under CWE-22 Path Traversal and aligns with ATT&CK techniques for privilege escalation and persistence through file system manipulation. The vulnerability demonstrates a critical failure in input validation and path handling that directly impacts the security of applications relying on this library for decompression operations.