CVE-2021-33644 in libtar
Summary
by MITRE • 08/11/2022
An attacker who submits a crafted tar file with size in header struct being 0 may be able to trigger an calling of malloc(0) for a variable gnu_longname, causing an out-of-bounds read.
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Analysis
by VulDB Data Team • 01/04/2026
The vulnerability identified as CVE-2021-33644 represents a critical security flaw in tar archive processing software that manifests through improper validation of archive header structures. This issue occurs when a maliciously crafted tar file contains a header with a size field set to zero, which triggers unexpected behavior in the archive extraction routine. The vulnerability specifically targets the GNU tar implementation and demonstrates a classic example of insufficient input validation that can lead to memory corruption and potential exploitation. The flaw resides in how the software handles the gnu_longname variable during archive processing, where the zero-sized header field causes the system to attempt allocation of memory with a size parameter of zero, creating a dangerous execution path.
The technical implementation of this vulnerability stems from the tar archive format's handling of extended header information, particularly when processing GNU-specific extensions. When the size field within the tar header structure equals zero, the processing logic fails to properly validate this condition before proceeding with memory allocation operations. This failure creates a scenario where malloc(0) is invoked, which while not immediately crashing the application, can lead to undefined behavior and memory layout issues that may be exploited by adversaries. The gnu_longname variable becomes a critical point of failure because it represents a mechanism for storing extended file names that exceed standard tar header limitations, yet the validation logic fails to properly handle the edge case of zero-sized entries.
Operationally, this vulnerability presents significant risks to systems that process untrusted tar archives, particularly in automated build environments, file transfer systems, and any application that accepts tar files from external sources without proper sanitization. The out-of-bounds read condition can potentially expose sensitive memory contents, provide attackers with information about memory layout, and in some cases may be leveraged as a stepping stone for more sophisticated attacks. The vulnerability is particularly concerning in environments where tar files are processed automatically without user intervention, as it can be exploited through simple file upload mechanisms or automated download processes. Attackers can craft malicious tar files that appear legitimate but contain the zero-sized header field to trigger the problematic code path.
From a cybersecurity perspective, this vulnerability aligns with CWE-129, which addresses improper validation of input bounds, and can be categorized under the ATT&CK technique T1059.007 for execution through archive files. The flaw demonstrates how seemingly benign input validation issues can create significant security implications in archive processing software. Organizations using affected tar implementations should immediately implement mitigations including input sanitization, validation of header fields, and proper bounds checking before memory allocation operations. The recommended approach involves updating to patched versions of the tar software, implementing strict header validation routines, and potentially using alternative archive processing libraries that have been audited for similar vulnerabilities. Additionally, organizations should consider implementing network-level controls to restrict the processing of untrusted archive files and establish robust monitoring for unusual memory allocation patterns that might indicate exploitation attempts.
The broader implications of this vulnerability extend beyond immediate exploitation potential to highlight the importance of comprehensive input validation in archive processing systems. This flaw demonstrates how the interaction between different header fields and memory management operations can create unexpected security implications, particularly in legacy software that may not have been designed with modern security considerations in mind. System administrators and security professionals should treat this vulnerability as a high-priority issue requiring immediate attention, especially in environments where automated processing of archive files occurs without proper security controls. The vulnerability serves as a reminder of the critical need for thorough testing of edge cases in security-sensitive software components, particularly those dealing with file format parsing and memory management operations.