CVE-2011-4137 in Django
Summary
by MITRE
The verify_exists functionality in the URLField implementation in Django before 1.2.7 and 1.3.x before 1.3.1 relies on Python libraries that attempt access to an arbitrary URL with no timeout, which allows remote attackers to cause a denial of service (resource consumption) via a URL associated with (1) a slow response, (2) a completed TCP connection with no application data sent, or (3) a large amount of application data, a related issue to CVE-2011-1521.
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Analysis
by VulDB Data Team • 11/24/2021
The vulnerability described in CVE-2011-4137 represents a critical denial of service weakness in Django's URLField implementation that existed in versions prior to 1.2.7 and 1.3.1. This flaw specifically targets the verify_exists functionality which is designed to validate whether a given URL actually exists and is accessible. The vulnerability arises from the underlying Python libraries that Django utilizes for URL verification, which lack proper timeout mechanisms when attempting to establish connections to remote resources. This absence of timeout controls creates an exploitable condition where malicious actors can craft specially crafted URLs that will cause the application to hang indefinitely while waiting for responses.
The technical implementation of this vulnerability stems from the improper handling of network connections within Django's URL validation process. When the verify_exists parameter is enabled on a URLField, the framework attempts to establish connections to verify the existence of URLs through standard Python networking libraries. These libraries perform blocking network operations without any configured timeouts, meaning that the application thread will wait indefinitely for network responses. The vulnerability manifests under three specific conditions that all lead to resource exhaustion: first, when a URL responds slowly enough to exceed any reasonable timeout threshold, second, when a TCP connection completes but sends no application data, and third, when a URL returns an excessive amount of data that consumes available memory resources. This behavior directly maps to CWE-400, which describes unchecked resource consumption vulnerabilities in software systems.
From an operational perspective, this vulnerability presents a significant threat to Django applications that utilize the verify_exists functionality in their URLField implementations. Attackers can exploit this weakness by providing malicious URLs that either deliberately slow response times or return massive data payloads, effectively causing the application servers to become unresponsive and consume all available resources. The impact extends beyond simple service disruption as the vulnerability can be leveraged to exhaust system resources including memory, CPU cycles, and network bandwidth, potentially leading to complete application failure and system instability. This type of attack aligns with ATT&CK technique T1499.004, which focuses on network denial of service attacks targeting resource exhaustion. The vulnerability is particularly dangerous because it can be triggered through normal user input validation processes, making it difficult to distinguish between legitimate and malicious requests.
The mitigation strategy for CVE-2011-4137 requires immediate patching of affected Django versions to the recommended secure releases. Organizations should upgrade their Django installations to versions 1.2.7 or 1.3.1 and later, which contain the necessary fixes to implement proper timeout mechanisms for URL verification operations. Additionally, system administrators should consider disabling the verify_exists functionality on URLFields when it is not strictly required, as this eliminates the vulnerable code path entirely. Implementing network-level controls such as connection timeouts and bandwidth limiting can provide additional defense in depth. The fix addresses the root cause by ensuring that all network operations within the URL verification process include appropriate timeout parameters, preventing indefinite blocking of application threads. This remediation approach directly addresses the underlying CWE-400 vulnerability by implementing proper resource management and timeout controls in network operations.