CVE-2021-33643 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_longlink, causing an out-of-bounds read.
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Analysis
by VulDB Data Team • 06/23/2026
The vulnerability identified as CVE-2021-33643 represents a critical security flaw in tar archive processing software that can lead to memory corruption and potential arbitrary code execution. This issue manifests when a maliciously crafted tar file contains a header structure with a size field set to zero, specifically affecting the gnu_longlink variable during archive extraction. The flaw exists in the handling of tar file headers where the software fails to properly validate the size field before attempting memory allocation operations, creating a pathway for attackers to manipulate the extraction process.
The technical implementation of this vulnerability stems from improper input validation within the tar archive parsing logic. When the tar extraction utility encounters a header with a zero size field, it proceeds to call malloc(0) for the gnu_longlink variable without adequate bounds checking. This malloc(0) call typically returns a valid pointer to a zero-sized memory block, but the subsequent operations that attempt to read beyond this allocated space result in out-of-bounds memory access patterns. The vulnerability is categorized under CWE-129 as "Improper Validation of Array Index" and specifically relates to improper handling of edge cases in memory allocation routines. The flaw demonstrates characteristics consistent with heap-based buffer overflows and memory corruption issues that can be exploited through crafted input files.
The operational impact of CVE-2021-33643 extends beyond simple denial of service scenarios, as the out-of-bounds read conditions can potentially be leveraged to disclose sensitive memory contents or even facilitate more sophisticated exploitation techniques. Attackers can craft malicious tar archives that, when processed by vulnerable software, trigger memory access violations that may reveal stack or heap contents, potentially exposing sensitive data or aiding in further exploitation attempts. The vulnerability affects systems that process untrusted tar archives, including build systems, backup utilities, and any software that extracts tar files without proper input sanitization. This makes it particularly dangerous in environments where automated processing of user-uploaded content occurs, as the attack surface expands significantly.
Mitigation strategies for this vulnerability require immediate patching of affected tar processing libraries and applications, with particular attention to ensuring proper validation of header size fields before memory allocation operations. Organizations should implement input sanitization measures that validate all tar header fields, including size fields, against expected ranges and reject archives with suspicious or malformed headers. The implementation of address space layout randomization and stack canaries can provide additional defense-in-depth measures, though these are secondary protections to the primary input validation fixes. System administrators should also consider implementing automated scanning of tar archives for suspicious patterns and establishing secure extraction practices that limit the privileges of archive processing utilities. The vulnerability aligns with ATT&CK technique T1059.007 for execution through archive extraction and T1566 for social engineering through malicious file delivery, making it particularly relevant for organizations implementing comprehensive threat detection and response strategies.