CVE-2020-8492 in Python
Summary
by MITRE
Python 2.7 through 2.7.17, 3.5 through 3.5.9, 3.6 through 3.6.10, 3.7 through 3.7.6, and 3.8 through 3.8.1 allows an HTTP server to conduct Regular Expression Denial of Service (ReDoS) attacks against a client because of urllib.request.AbstractBasicAuthHandler catastrophic backtracking.
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Analysis
by VulDB Data Team • 03/27/2024
The vulnerability identified as CVE-2020-8492 represents a critical security flaw in Python's urllib.request module that enables remote attackers to execute Regular Expression Denial of Service (ReDoS) attacks against clients. This issue affects multiple Python versions including 2.7 through 2.7.17, 3.5 through 3.5.9, 3.6 through 3.6.10, 3.7 through 3.7.6, and 3.8 through 3.8.1, making it a widespread concern across the Python ecosystem. The vulnerability specifically resides within the AbstractBasicAuthHandler class which processes HTTP authentication headers containing regular expressions that are susceptible to catastrophic backtracking patterns.
The technical flaw manifests when a malicious HTTP server responds to a client request with a specially crafted authentication header that contains a regular expression pattern designed to cause exponential backtracking in the Python regex engine. This occurs because the AbstractBasicAuthHandler class uses regular expressions to parse authentication credentials, and when these patterns contain nested quantifiers with overlapping matches, the regex engine can spend exponentially increasing amounts of time attempting to match the input. The vulnerability is classified as CWE-400 under the Common Weakness Enumeration, specifically targeting "Uncontrolled Resource Consumption" where the regex matching process consumes excessive CPU cycles and memory resources, effectively creating a denial of service condition.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited by remote attackers to consume system resources indefinitely, causing applications to become unresponsive or crash entirely. When a Python application using urllib.request connects to a malicious server and receives an authentication header with the vulnerable regex pattern, the client's processing time increases exponentially with the length of the input string, potentially leading to complete system exhaustion. This vulnerability directly maps to ATT&CK technique T1499.004 which involves Network Denial of Service through resource exhaustion, and also aligns with T1595.001 for reconnaissance activities where attackers might discover and exploit such weaknesses in network infrastructure components.
Mitigation strategies for CVE-2020-8492 include upgrading to patched versions of Python where the vulnerable regular expressions have been either replaced with more efficient alternatives or rewritten to prevent catastrophic backtracking. Organizations should also implement network-level protections such as rate limiting and input validation on authentication headers to prevent exploitation. Additionally, security teams should consider implementing application firewalls or web application firewalls that can detect and block suspicious authentication header patterns. The most effective long-term solution involves upgrading to Python 3.8.2 or later versions where the vulnerability has been resolved through improved regex handling in the urllib.request module, ensuring that the AbstractBasicAuthHandler no longer employs vulnerable regular expression patterns that could lead to catastrophic backtracking scenarios.