CVE-2009-3695 in Django
Summary
by MITRE
Algorithmic complexity vulnerability in the forms library in Django 1.0 before 1.0.4 and 1.1 before 1.1.1 allows remote attackers to cause a denial of service (CPU consumption) via a crafted (1) EmailField (email address) or (2) URLField (URL) that triggers a large amount of backtracking in a regular expression.
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Analysis
by VulDB Data Team • 08/23/2021
The vulnerability identified as CVE-2009-3695 represents a critical algorithmic complexity issue within Django's forms library that enables remote attackers to execute denial of service attacks through carefully crafted input data. This flaw specifically targets the validation mechanisms employed by EmailField and URLField components, exploiting inherent weaknesses in regular expression processing that can lead to excessive cpu consumption and system instability.
The technical root cause of this vulnerability lies in the implementation of regular expressions used for email and URL validation within Django's form processing framework. When maliciously crafted input is provided to these fields, the regular expressions undergo massive backtracking operations that exponentially increase processing time. This behavior creates a condition where the validation process consumes disproportionate computational resources, effectively allowing an attacker to exhaust system cpu cycles and render the application unresponsive. The vulnerability manifests when the input string triggers the regular expression engine to perform an exponential number of operations, a well-documented pattern that aligns with CWE-1333, which specifically addresses algorithmic complexity vulnerabilities in regular expressions.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire application availability. Attackers can exploit this weakness by submitting carefully constructed email addresses or URLs that cause the validation routines to enter into prolonged computational loops. The effects are particularly severe in web applications where form validation occurs on every user interaction, as this vulnerability can be leveraged to create sustained denial of service conditions that are difficult to distinguish from legitimate high-traffic scenarios. This type of attack directly maps to the ATT&CK technique T1499.004, which describes denial of service via resource exhaustion, making it a significant concern for web application security.
The vulnerability affects Django versions prior to 1.0.4 and 1.1.1, indicating that this was a widespread issue that impacted numerous applications relying on the framework's form validation capabilities. Organizations running affected versions were exposed to potential exploitation through any application that utilized EmailField or URLField components in their forms, creating a substantial attack surface across the django ecosystem. The remediation strategy involves upgrading to patched versions of Django where the regular expressions have been optimized to prevent backtracking attacks, typically through the implementation of more efficient validation patterns or limiting the maximum backtracking depth allowed during regex processing.
Security practitioners should recognize this vulnerability as a classic example of regular expression denial of service attacks, which have been documented across various frameworks and applications. The fix implemented in Django's subsequent releases demonstrates the importance of proper input validation design and the need for security-conscious development practices. This vulnerability underscores the necessity of implementing input validation that considers computational complexity and resource consumption, particularly when dealing with user-supplied data that undergoes pattern matching operations. Organizations should also implement monitoring and rate-limiting mechanisms to detect and mitigate potential exploitation attempts, while maintaining current security patches to protect against similar algorithmic complexity issues that may arise in other components of their application stack.