CVE-2021-3733 in Python
Summary
by MITRE • 03/10/2022
There's a flaw in urllib's AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client. The greatest threat that this flaw poses is to application availability.
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Analysis
by VulDB Data Team • 11/04/2025
The vulnerability identified as CVE-2021-3733 resides within Python's urllib library, specifically in the AbstractBasicAuthHandler class that manages HTTP basic authentication. This flaw represents a critical security weakness that can be exploited through Regular Expression Denial of Service (ReDOS) attacks. The vulnerability manifests when an HTTP client application connects to a malicious server that sends a specially crafted response containing malicious regular expression patterns. When the client's urllib library processes this authentication response, the regular expression engine becomes vulnerable to a denial of service attack that can consume excessive CPU resources and potentially crash the application.
The technical implementation of this vulnerability involves the improper handling of authentication headers within the AbstractBasicAuthHandler class. When a client receives an authentication challenge from a server, the library attempts to parse and validate the authentication response using regular expressions. An attacker can craft a malicious response that contains regular expression patterns designed to cause catastrophic backtracking in the regex engine. This creates a scenario where the regular expression matching algorithm becomes exponentially slow as it tries to process the malformed input, leading to resource exhaustion and application unresponsiveness. The vulnerability specifically impacts Python applications that use urllib for HTTP communication and authentication handling.
The operational impact of CVE-2021-3733 extends beyond simple service disruption to potentially compromise application availability and system stability. Applications that rely on urllib for HTTP operations, including web browsers, automated clients, and network utilities, become vulnerable to this attack vector. The threat is particularly severe because it can be triggered through normal network operations without requiring any special privileges from the attacker. The attack can be executed through various means including malicious web servers, compromised network infrastructure, or even through man-in-the-middle attacks that redirect traffic to malicious endpoints. This vulnerability affects the availability of applications and can be used to create persistent denial of service conditions that are difficult to detect and mitigate.
Security mitigations for CVE-2021-3733 primarily involve updating to patched versions of Python where the regular expression handling has been improved to prevent catastrophic backtracking. Organizations should prioritize patch management for Python installations and ensure that all applications using urllib are updated to versions that address this vulnerability. Additionally, network administrators can implement defensive measures such as rate limiting and monitoring for unusual CPU usage patterns that might indicate a ReDOS attack in progress. The vulnerability aligns with CWE-400 which specifically addresses unchecked resource consumption, and represents a variant of the broader ReDOS attack pattern categorized under ATT&CK technique T1496 for resource exhaustion. Organizations should also consider implementing input validation and sanitization measures to reduce the attack surface and monitor for suspicious authentication response patterns that might indicate exploitation attempts.