CVE-2017-18361 in Colander
Summary
by MITRE
In Pylons Colander through 1.6, the URL validator allows an attacker to potentially cause an infinite loop thereby causing a denial of service via an unclosed parenthesis.
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Analysis
by VulDB Data Team • 09/08/2025
The vulnerability identified as CVE-2017-18361 affects Pylons Colander version 1.6 and earlier, representing a denial of service weakness that stems from improper handling of URL validation logic. This issue specifically manifests when the URL validator encounters malformed input containing an unclosed parenthesis, creating a condition where the validation process enters an infinite loop. The flaw exists within the validation algorithm's inability to properly terminate parsing operations when encountering certain syntactic structures that should be rejected as invalid URLs.
The technical implementation of this vulnerability resides in the URL validator's parsing mechanism which fails to account for unbalanced parentheses in URL strings. When processing a URL containing an unclosed parenthesis, the validator's internal state management becomes compromised, leading to recursive or iterative processing that never reaches a termination condition. This behavior directly maps to CWE-835, which describes the weakness of infinite loops or iterations that do not properly terminate, and represents a classic example of a resource exhaustion attack vector. The vulnerability demonstrates how seemingly benign input validation can be exploited to consume system resources indefinitely.
The operational impact of this vulnerability extends beyond simple service disruption, as it provides attackers with a straightforward method to exhaust system resources through a relatively simple input manipulation. An attacker can craft malicious URLs containing unclosed parentheses and submit them to applications using Colander's URL validation, causing the application to hang or consume excessive CPU cycles. This creates a denial of service condition that can affect web applications, API endpoints, and any system component relying on Colander's URL validation for input sanitization. The attack requires minimal sophistication and can be automated, making it particularly dangerous in high-traffic environments where resource exhaustion can cascade into broader system failures.
Mitigation strategies for CVE-2017-18361 should prioritize updating to Pylons Colander version 1.7 or later, where the URL validator has been patched to properly handle malformed input structures. Organizations should implement input validation at multiple layers including application-level defenses, web application firewalls, and rate limiting mechanisms to prevent exploitation attempts. The fix addresses the root cause by ensuring proper termination conditions in the URL parsing algorithm and includes enhanced error handling for malformed syntactic structures. Security teams should also consider implementing monitoring for unusual CPU usage patterns or hanging processes that might indicate exploitation attempts, as this vulnerability can be used in conjunction with other attack vectors to create more sophisticated denial of service scenarios.