CVE-2026-4426 in libarchive
Summary
by MITRE • 03/19/2026
A flaw was found in libarchive. An Undefined Behavior vulnerability exists in the zisofs decompression logic, caused by improper validation of a field (`pz_log2_bs`) read from ISO9660 Rock Ridge extensions. A remote attacker can exploit this by supplying a specially crafted ISO file. This can lead to incorrect memory allocation and potential application crashes, resulting in a denial-of-service (DoS) condition.
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Analysis
by VulDB Data Team • 05/22/2026
The vulnerability identified as CVE-2026-4426 resides within the libarchive library, a widely-used software component for handling various archive formats including iso9660 filesystems. This flaw manifests as an undefined behavior condition that occurs during the decompression process of zisofs (zipped iso filesystem) data structures. The root cause stems from insufficient validation of metadata fields extracted from ISO9660 Rock Ridge extensions, specifically the `pz_log2_bs` field which represents the logarithm base 2 of the block size. When processing maliciously crafted iso files, the library fails to properly validate this field before using it in memory allocation calculations, creating a pathway for exploitation that can be leveraged remotely.
The technical implementation of this vulnerability involves the improper handling of integer values derived from the Rock Ridge extension data structure within iso9660 filesystems. The `pz_log2_bs` field contains a value that should represent a valid block size logarithm, but when this value is not properly validated, it can lead to arithmetic overflow or underflow conditions during memory allocation operations. This undefined behavior can cause the application to allocate incorrect amounts of memory or access invalid memory regions, ultimately leading to segmentation faults or application crashes. The vulnerability specifically affects the zisofs decompression logic where the library attempts to allocate memory buffers based on the unvalidated block size parameter.
From an operational perspective, this vulnerability presents a significant denial-of-service risk to systems that utilize libarchive for processing iso files from untrusted sources. Attackers can craft specially formatted iso files containing maliciously constructed Rock Ridge extensions that trigger the undefined behavior when the library attempts to decompress zisofs data. The impact extends beyond simple application crashes as the vulnerability can be exploited in automated scanning systems, web applications, or file processing services that handle user-uploaded iso files. This makes it particularly dangerous in environments where automated processing of external file formats is common, potentially allowing attackers to disrupt services through sustained DoS attacks or to cause cascading failures in dependent systems.
Mitigation strategies for this vulnerability should prioritize immediate patching of affected libarchive versions, as the fix typically involves implementing proper validation of the `pz_log2_bs` field before using it in memory allocation calculations. System administrators should also implement strict input validation for iso files, particularly those received from untrusted sources, and consider deploying sandboxed environments for processing potentially malicious archive files. Network-level protections can include filtering iso files at ingress points and implementing rate limiting for file processing operations. The vulnerability aligns with CWE-704 Undefined Behavior in a library context and maps to ATT&CK technique T1499.004 for denial-of-service attacks through application vulnerabilities, emphasizing the need for comprehensive security measures beyond simple patch management to protect against exploitation attempts.
This vulnerability demonstrates the critical importance of proper input validation in cryptographic and archive processing libraries, where malformed data can lead to severe operational consequences. The complexity of iso9660 filesystem structures and Rock Ridge extensions creates numerous potential attack vectors within archive processing systems, making thorough validation of all extracted metadata fields essential for maintaining system integrity and availability.