CVE-2019-20907 in Python
Summary
by MITRE
In Lib/tarfile.py in Python through 3.8.3, an attacker is able to craft a TAR archive leading to an infinite loop when opened by tarfile.open, because _proc_pax lacks header validation.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 04/30/2025
The vulnerability identified as CVE-2019-20907 represents a critical denial of service flaw within Python's tarfile module that affects versions through 3.8.3. This issue stems from insufficient input validation within the _proc_pax method, which processes pax extended headers in tar archives. The flaw specifically manifests when an attacker constructs a malicious tar archive containing malformed pax headers that trigger an infinite loop during the archive opening process. This vulnerability falls under the category of CWE-835, which describes loops that are not properly terminated, creating conditions where the processing routine can become trapped in an endless cycle.
The technical implementation of this vulnerability exploits the tarfile module's handling of pax headers, which are extended header formats used in tar archives to store additional metadata beyond the standard tar format limitations. When tarfile.open encounters a tar archive with crafted pax headers, the _proc_pax function fails to validate the header structure properly, leading to a scenario where the parsing logic continuously iterates without making progress toward completing the archive processing. This condition effectively creates an infinite loop that consumes system resources and prevents normal archive operations from completing successfully. The vulnerability is particularly concerning because it can be triggered simply by attempting to open a maliciously crafted tar file, requiring no special privileges or complex attack vectors.
The operational impact of CVE-2019-20907 extends beyond simple resource exhaustion, as it can be leveraged in various attack scenarios that exploit the denial of service nature of the flaw. Systems that automatically process or validate tar archives, such as automated build systems, package managers, or content delivery platforms, become vulnerable to this attack vector. The infinite loop behavior can cause processes to hang indefinitely, leading to service unavailability and potentially cascading failures in systems that depend on tar archive processing. This vulnerability directly aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a classic example of how malformed input can be exploited to create system instability. The flaw can be particularly dangerous in environments where tar archives are processed automatically without proper input validation, as it allows attackers to disrupt services with minimal effort.
Mitigation strategies for CVE-2019-20907 primarily involve updating to Python versions 3.8.4 or later, where the vulnerability has been addressed through improved input validation in the _proc_pax method. Organizations should implement comprehensive patch management procedures to ensure all systems processing tar archives are updated promptly. Additionally, defensive measures such as implementing proper input validation at the application level, using sandboxed environments for archive processing, and monitoring for unusual resource consumption patterns can help detect and prevent exploitation attempts. The fix implemented in Python 3.8.4 specifically addresses the missing header validation by adding proper bounds checking and loop termination conditions within the pax header processing logic, ensuring that malformed headers cannot trigger the infinite loop condition. System administrators should also consider implementing network-level controls that restrict access to potentially malicious archives and establish automated monitoring for processes that become unresponsive during archive operations.