CVE-2013-7338 in Python
Summary
by MITRE
Python before 3.3.4 RC1 allows remote attackers to cause a denial of service (infinite loop and CPU consumption) via a file size value larger than the size of the zip file to the (1) ZipExtFile.read, (2) ZipExtFile.read(n), (3) ZipExtFile.readlines, (4) ZipFile.extract, or (5) ZipFile.extractall function.
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Analysis
by VulDB Data Team • 05/11/2026
The vulnerability identified as CVE-2013-7338 represents a critical denial of service flaw affecting Python versions prior to 3.3.4 RC1. This issue specifically targets the zip file handling mechanisms within the Python standard library, creating a condition where maliciously crafted zip file parameters can trigger infinite loops and excessive cpu consumption. The vulnerability stems from insufficient validation of file size parameters during zip file extraction operations, allowing attackers to manipulate the system's resource allocation through carefully constructed input values.
The technical implementation of this vulnerability occurs within the ZipExtFile and ZipFile classes of Python's standard library. When attackers provide a file size value that exceeds the actual size of the zip file, the reading functions read readlines, extract, and extractall become susceptible to entering infinite loops. This happens because the internal logic fails to properly validate the relationship between the requested read size and the available file data, causing the system to continuously attempt to read beyond the actual file boundaries. The flaw operates at the level of stream processing where the zip file reader attempts to maintain proper positioning and data retrieval, but becomes trapped in circular logic when confronted with inconsistent size parameters.
The operational impact of this vulnerability extends beyond simple resource exhaustion, as it can be exploited to create sustained denial of service conditions that may affect web applications, automated systems, or any service that processes zip files through Python. Attackers can craft zip files with oversized size parameters that cause the target system to consume excessive cpu cycles and memory resources, potentially leading to system instability, application crashes, or complete service unavailability. The vulnerability is particularly concerning in environments where zip file processing is automated or part of regular workflows, as it can be triggered without requiring authentication or specialized privileges.
This vulnerability maps to CWE-129 Input Validation and OWASP Top Ten category A03: Injection, specifically addressing issues related to improper input validation during file processing operations. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under T1499.004 Network Denial of Service, where adversaries exploit software weaknesses to consume system resources. The vulnerability demonstrates how seemingly benign file processing operations can become attack vectors when proper input sanitization and boundary checking are omitted. Organizations relying on Python applications for file handling should consider this vulnerability as part of their broader security posture assessment, particularly in environments where untrusted zip files may be processed.
Mitigation strategies for CVE-2013-7338 require immediate patching of affected Python installations to version 3.3.4 RC1 or later, which includes proper bounds checking and validation of file size parameters. System administrators should implement input validation measures at the application level, ensuring that all zip file processing operations include proper size parameter verification before execution. Additionally, implementing resource limits and timeouts on file processing operations can help prevent exploitation from causing complete system degradation. Organizations should also consider deploying network monitoring solutions to detect unusual cpu consumption patterns that may indicate exploitation attempts. Regular security assessments and vulnerability scanning should include verification of Python library versions to prevent similar issues from remaining undetected in production environments.