CVE-2019-9923 in tar
Summary
by MITRE
pax_decode_header in sparse.c in GNU Tar before 1.32 had a NULL pointer dereference when parsing certain archives that have malformed extended headers.
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Analysis
by VulDB Data Team • 08/07/2025
The vulnerability identified as CVE-2019-9923 affects GNU Tar version 1.31 and earlier, specifically within the pax_decode_header function located in sparse.c. This flaw represents a critical NULL pointer dereference that occurs during the parsing of archive files containing malformed extended headers. The issue stems from insufficient input validation and error handling within the archive parsing mechanism, creating a scenario where maliciously crafted archive files can trigger unexpected program termination through segmentation faults.
The technical implementation of this vulnerability involves the pax_decode_header function failing to properly validate header structures before attempting to access memory locations that may be NULL. When GNU Tar encounters extended headers with malformed data structures, the function attempts to dereference pointers that have not been properly initialized or validated, leading to a crash condition. This behavior aligns with CWE-476 which specifically addresses NULL pointer dereference vulnerabilities, and demonstrates how improper input validation can lead to denial of service conditions. The vulnerability is particularly concerning because it can be triggered through normal archive processing operations without requiring special privileges or complex exploitation techniques.
Operationally, this vulnerability presents significant risks to systems that regularly process untrusted archive files, particularly in environments where automated backup processes, file distribution systems, or continuous integration pipelines rely on GNU Tar for archive management. An attacker could exploit this by crafting malicious archive files containing malformed extended headers that would cause GNU Tar to crash when processing these files. The impact extends beyond simple service disruption as this vulnerability could be leveraged in broader attack chains where denial of service conditions might be used to facilitate other exploits or simply disrupt legitimate system operations. The ATT&CK framework categorizes this type of vulnerability under T1499 which covers resource exhaustion and T1059 which involves command and scripting interpreter usage for exploitation.
Mitigation strategies for CVE-2019-9923 primarily involve upgrading to GNU Tar version 1.32 or later where the vulnerability has been patched. System administrators should prioritize updating tar packages across all systems that process external archive files, particularly those in high-risk environments such as web servers, file sharing systems, and automated processing pipelines. Additional protective measures include implementing strict input validation for archive files, using sandboxed environments for archive processing, and deploying automated monitoring to detect unusual tar process behavior. Organizations should also consider implementing network segmentation to limit exposure and establish incident response procedures specifically addressing archive processing vulnerabilities. The patch addresses the core issue by adding proper NULL pointer checks and enhanced error handling within the pax_decode_header function, preventing the dereference of uninitialized memory locations while maintaining compatibility with legitimate archive formats.