CVE-2018-7536 in Django
Summary
by MITRE
An issue was discovered in Django 2.0 before 2.0.3, 1.11 before 1.11.11, and 1.8 before 1.8.19. The django.utils.html.urlize() function was extremely slow to evaluate certain inputs due to catastrophic backtracking vulnerabilities in two regular expressions (only one regular expression for Django 1.8.x). The urlize() function is used to implement the urlize and urlizetrunc template filters, which were thus vulnerable.
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Analysis
by VulDB Data Team • 02/21/2023
The vulnerability identified as CVE-2018-7536 represents a critical performance degradation issue within the Django web framework that affects multiple versions including 2.0 before 2.0.3, 1.11 before 1.11.11, and 1.8 before 1.8.19. This flaw specifically targets the django.utils.html.urlize() function which serves as the underlying mechanism for processing URLs within Django applications. The issue manifests through catastrophic backtracking in regular expressions used by the urlize() function, creating a scenario where certain maliciously crafted inputs can cause the function to consume excessive computational resources and potentially lead to denial of service conditions. The vulnerability operates at the core of Django's template processing system since the urlize and urlizetrunc template filters directly utilize this vulnerable function, making it accessible through standard template rendering operations.
The technical root cause of this vulnerability stems from poorly constructed regular expressions that exhibit exponential time complexity when processing specific input patterns. This type of vulnerability maps directly to CWE-400, which classifies improper handling of regular expressions that can lead to resource exhaustion. When the urlize() function encounters input strings that trigger backtracking in its regular expressions, the computational complexity grows exponentially rather than linearly, causing the function to take an extraordinarily long time to process even relatively small inputs. The backtracking occurs when the regular expression engine attempts to match patterns that contain nested quantifiers or alternations that create multiple possible matching paths, leading to a combinatorial explosion in the number of potential matches that must be evaluated. This behavior is particularly dangerous in web applications where user input flows directly into the urlize function, as attackers can craft inputs that cause the application to become unresponsive or crash.
The operational impact of CVE-2018-7536 extends beyond simple performance degradation to potentially compromise the availability and stability of Django applications. Attackers can exploit this vulnerability by submitting carefully crafted URLs or text containing specific patterns that trigger the backtracking behavior, causing the application server to become unresponsive for extended periods. This creates a denial of service condition that can affect legitimate users and potentially allow attackers to exhaust system resources or cause application crashes. The vulnerability affects both the urlize and urlizetrunc template filters, which are commonly used throughout Django applications for rendering hyperlinks and truncated URLs, making the impact widespread across various application components. In practical terms, this vulnerability can be exploited through any user input that gets processed through these template filters, including form submissions, API requests, or even content management system inputs where URL processing is performed.
Mitigation strategies for CVE-2018-7536 primarily focus on upgrading to patched versions of Django where the regular expressions have been corrected to prevent catastrophic backtracking scenarios. Organizations should immediately update their Django installations to versions 2.0.3, 1.11.11, or 1.8.19 respectively, as these releases contain the necessary fixes to address the vulnerable regular expression patterns. Additionally, implementing input validation and sanitization at the application level can provide defense-in-depth measures, particularly when processing untrusted user input that might eventually reach the urlize function. The ATT&CK framework categorizes this vulnerability under T1499.004 for resource exhaustion attacks and T1595.001 for reconnaissance techniques, as attackers may first probe systems to identify vulnerable Django versions before attempting exploitation. Network-level protections such as rate limiting and input length restrictions can also help mitigate the impact of this vulnerability by preventing attackers from submitting excessively long inputs that trigger the backtracking behavior. Organizations should also consider implementing monitoring and alerting mechanisms to detect unusual processing times or resource consumption patterns that might indicate exploitation attempts.