CVE-2021-33503 in urllib3
Summary
by MITRE • 06/29/2021
An issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the authority regular expression exhibits catastrophic backtracking, causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.
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Analysis
by VulDB Data Team • 07/03/2021
The vulnerability identified as CVE-2021-33503 represents a critical denial of service flaw within the urllib3 library, a fundamental component of python applications that handle http requests. This issue specifically affects versions prior to 1.26.5 and stems from improper handling of URL parsing logic when encountering malformed input containing excessive characters within the authority portion of a URL. The problem manifests when urllib3 processes URLs where the authority component contains numerous symbols, creating a scenario that triggers catastrophic backtracking in the regular expression engine responsible for parsing these components.
The technical root cause of this vulnerability lies in the regular expression pattern used to parse the authority section of URLs within urllib3's internal parsing mechanisms. When a URL contains many characters in its authority component, the regular expression engine enters a state of exponential backtracking where it repeatedly attempts different matching paths through the pattern, ultimately consuming massive amounts of processing time and system resources. This behavior directly corresponds to CWE-400 vulnerability category, which encompasses issues related to uncontrolled resource consumption that can lead to denial of service conditions. The regular expression implementation fails to properly handle the ambiguous matching scenarios created by multiple characters, causing the parsing routine to spiral into computational exhaustion.
The operational impact of this vulnerability extends beyond simple resource exhaustion, creating potential security implications for applications that process user-provided URLs or handle HTTP redirects. Attackers can exploit this weakness by crafting malicious URLs containing excessive @ characters, which when processed by vulnerable urllib3 versions can cause applications to become unresponsive or crash entirely. This vulnerability is particularly dangerous in web applications that accept URL parameters, handle redirects, or process user input containing URLs, as it allows for remote denial of service attacks without requiring authentication or specialized privileges. The attack vector becomes more pronounced when applications redirect users to URLs without proper validation, as the vulnerability can be triggered during the redirect processing phase.
Mitigation strategies for CVE-2021-33503 primarily focus on upgrading to urllib3 version 1.26.5 or later, where the regular expression patterns have been properly refactored to prevent catastrophic backtracking scenarios. Organizations should conduct thorough vulnerability assessments to identify all applications and systems using vulnerable urllib3 versions, particularly those handling user input or HTTP redirects. Additional protective measures include implementing proper URL validation at application boundaries, sanitizing URL parameters before processing, and deploying input validation mechanisms that can detect and reject malformed URLs containing excessive characters. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 (Network Denial of Service) and T1595.001 (Network Reconnaissance) as attackers can leverage it to disrupt services and potentially gather information about target applications. The fix implemented in version 1.26.5 addresses the core parsing logic by using more efficient regular expression patterns that do not exhibit backtracking behavior when processing URLs with multiple characters, thereby preventing the computational exhaustion that previously occurred during URL parsing operations.