CVE-2011-4957 in WordPress
Summary
by MITRE
The make_clickable function in wp-includes/formatting.php in WordPress before 3.1.1 does not properly check URLs before passing them to the PCRE library, which allows remote attackers to cause a denial of service (crash) via a comment with a crafted URL that triggers many recursive calls.
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Analysis
by VulDB Data Team • 12/05/2021
The vulnerability identified as CVE-2011-4957 represents a critical denial of service weakness in WordPress core functionality that existed in versions prior to 3.1.1. This flaw resides within the make_clickable function located in the wp-includes/formatting.php file, which is responsible for automatically converting URLs found in text into clickable hyperlinks. The vulnerability stems from inadequate input validation and sanitization of URLs before they are processed by the Perl Compatible Regular Expressions (PCRE) library. When malicious users submit comments containing specially crafted URLs, these inputs trigger recursive pattern matching operations within the PCRE engine that can consume excessive system resources and ultimately lead to application crashes or service unavailability.
The technical exploitation of this vulnerability leverages the inherent characteristics of regular expression processing and the way WordPress handles URL conversion in user-generated content. The make_clickable function processes text to identify potential URLs and convert them into hyperlinks, but fails to implement proper bounds checking or recursion limits when dealing with malformed or specially constructed URLs. When such inputs are passed to the PCRE library, the regular expression engine begins executing complex pattern matching operations that can result in exponential time complexity behavior, creating a condition where a single malicious input can cause the system to enter an infinite or near-infinite recursive loop. This type of vulnerability falls under the CWE-129 weakness category, which specifically addresses improper input validation that can lead to resource exhaustion and denial of service conditions.
From an operational impact perspective, this vulnerability poses significant risks to WordPress installations that accept user comments or other forms of user-generated content containing URLs. Attackers can exploit this weakness by submitting malicious comments with crafted URLs that, when processed by the make_clickable function, cause the web server or application to consume excessive CPU cycles and memory resources. The resulting denial of service can affect not only the specific blog or website where the attack occurs but can also potentially impact server performance for other applications running on the same infrastructure. The vulnerability is particularly dangerous because it requires minimal technical expertise to exploit and can be executed through simple comment submissions, making it a preferred attack vector for automated or semi-automated denial of service campaigns.
The remediation strategy for CVE-2011-4957 involves upgrading to WordPress version 3.1.1 or later, which includes proper input validation and bounds checking within the make_clickable function. Security administrators should also implement additional protective measures such as rate limiting for comment submissions, input sanitization at multiple layers, and monitoring for unusual processing patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper regular expression usage and input validation in web applications, aligning with ATT&CK technique T1499.004 for resource exhaustion attacks. Organizations should also consider implementing web application firewalls and content filtering mechanisms to detect and block suspicious URL patterns before they can be processed by vulnerable functions. Regular security audits and code reviews focusing on regular expression handling and input validation practices can help identify similar vulnerabilities in other components of the application stack.