CVE-2018-1061 in Python
Summary
by MITRE
python before versions 2.7.15, 3.4.9, 3.5.6 and 3.7.0 is vulnerable to catastrophic backtracking in the difflib.IS_LINE_JUNK method. An attacker could use this flaw to cause denial of service.
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Analysis
by VulDB Data Team • 03/28/2023
The vulnerability identified as CVE-2018-1061 represents a critical security flaw in Python's difflib module that affects multiple Python versions including 2.7.14 and earlier, 3.4.8 and earlier, 3.5.5 and earlier, and 3.7.0 alpha releases. This vulnerability specifically targets the IS_LINE_JUNK method within the difflib library, which is commonly used for comparing and generating differences between text files or sequences. The issue manifests as catastrophic backtracking in the regular expression patterns used internally by this method, creating a potential denial of service attack vector that can be exploited by malicious actors.
The technical flaw stems from the implementation of regular expressions within the IS_LINE_JUNK method that fails to properly handle certain input patterns, leading to exponential time complexity during pattern matching operations. When an attacker supplies carefully crafted input data that triggers specific regex patterns, the difflib module enters a state of catastrophic backtracking where the regular expression engine performs an enormous number of redundant calculations. This behavior causes the Python interpreter to consume excessive CPU resources and memory, ultimately leading to system resource exhaustion and denial of service conditions that can affect applications relying on the difflib module for text comparison operations.
The operational impact of this vulnerability extends beyond simple denial of service scenarios as it affects any application or system that utilizes Python's difflib module for text processing, including version control systems, code comparison tools, automated testing frameworks, and various text processing applications. The vulnerability is particularly concerning because it can be triggered through seemingly benign input data, making it difficult to detect and prevent. Attackers can craft malicious input that causes the application to enter an infinite loop or consume all available system resources, effectively rendering the application unusable and potentially causing cascading failures in larger systems where multiple processes depend on the affected functionality.
Mitigation strategies for CVE-2018-1061 primarily focus on upgrading to patched versions of Python where the regular expression patterns have been corrected to prevent catastrophic backtracking scenarios. Organizations should prioritize immediate patching of all affected Python installations, with particular attention to systems running Python 2.7.14 or earlier, 3.4.8 or earlier, 3.5.5 or earlier, and any 3.7.0 alpha releases. Additionally, implementing input validation and sanitization measures can provide temporary protection while awaiting patches, though these approaches are not comprehensive solutions. Security teams should also consider monitoring applications for unusual CPU or memory consumption patterns that might indicate exploitation attempts. This vulnerability aligns with CWE-1287, which addresses catastrophic backtracking in regular expressions, and falls under ATT&CK technique T1499.004 for network denial of service attacks, highlighting the importance of proper regex implementation and input handling in preventing such security incidents.