CVE-2026-114104 in adm-zip
Summary
by MITRE • 10/21/2026
adm-zip before 0.6.1 contains an inconsistent entry resolution vulnerability that allows attackers to bypass content validation by supplying ZIP archives with duplicate entry names. Attackers can craft archives where getEntry() returns the last duplicate while extractAllTo() writes the first, causing unvalidated bytes to be written to disk.
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Analysis
by VulDB Data Team • 10/21/2026
The vulnerability identified in adm-zip versions prior to 0.6.1 represents a critical flaw in how the library handles ZIP archive extraction, specifically concerning the resolution of file entries with duplicate names within an archive structure. This issue stems from an inconsistency between two core functions: getEntry and extractAllTo. When processing a maliciously crafted ZIP file containing multiple files or directories that share identical internal paths, these methods exhibit divergent behavior regarding which entry they select for operation. The getEntry method typically returns the last occurrence of a duplicate name found in the archive structure, whereas the extractAllTo function iterates through entries and writes out what it identifies as the first instance based on its internal traversal logic or storage order. This discrepancy creates a significant security gap where the validation performed against one entry does not align with the file actually being written to the filesystem.
From a technical perspective, this inconsistency allows for path traversal-like attacks even if basic name sanitization is in place. An attacker can construct a ZIP archive that includes two entries with the same target filename on disk but different internal paths or metadata. If validation logic checks the entry returned by getEntry, it may pass because that specific instance appears safe. However, when extractAllTo executes, it writes out the other duplicate entry, which might contain malicious content such as shellcode, web shells, or scripts designed for further exploitation. Because the written file was not subjected to the same validation checks applied to the validated entry, unvalidated bytes are directly persisted to disk. This effectively bypasses any intended security controls that rely on verifying filename integrity before extraction.
The operational impact of this vulnerability is severe, particularly in applications that accept user-uploaded ZIP files for processing or storage. Attackers can exploit this flaw to achieve arbitrary file write capabilities within the context of the application process. Depending on the permissions and directory structure where adm-zip operates, this could lead to overwriting critical system configuration files, injecting malicious code into web-accessible directories, or corrupting existing data stores. In server-side environments, such as Node.js applications using this library for file management services, successful exploitation can result in remote code execution if the attacker can control the content of the injected duplicate entry and place it within an executable path. This undermines the integrity and confidentiality guarantees expected from secure archive handling libraries.
This vulnerability aligns with CWE-829, which describes Inclusion of a Function From an Untrusted Control Sphere, although more accurately it maps to CWE-671: Missing Information About Critical Elements for Security Mitigation or CWE-435: Improper Interaction Between Multiple Correctly Designed Features. The root cause is the lack of consistent state management between validation and execution phases when dealing with ambiguous inputs like duplicate filenames. In terms of threat modeling, this behavior facilitates unauthorized file writes that can be leveraged in subsequent attack chains, potentially leading to privilege escalation or persistent access depending on the deployment environment.
To mitigate this risk, organizations must immediately upgrade adm-zip to version 0.6.1 or later, where the developers have addressed the inconsistency by ensuring that both validation and extraction operations target the same entry instance consistently. For systems unable to update promptly due to legacy constraints, implementing a pre-processing step is recommended. This involves iterating through all entries in the ZIP archive before calling extractAllTo, identifying any duplicate filenames, and either rejecting the entire archive or renaming conflicting entries to unique names prior to validation and extraction. Additionally, applications should enforce strict allowlists for allowed file extensions and destination directories during the extraction process to limit the blast radius even if a bypass occurs. Regular security audits of third-party dependencies are essential to detect such subtle logic flaws that do not manifest as obvious crashes but rather as silent data integrity violations.