CVE-2018-14940 in phpCMS
Summary
by MITRE
PHPCMS 9 allows remote attackers to cause a denial of service (resource consumption) via large font_size, height, and width parameters in an api.php?op=checkcode request.
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Analysis
by VulDB Data Team • 03/13/2020
The vulnerability identified as CVE-2018-14940 affects PHPCMS version 9, a widely used content management system that has been targeted by attackers seeking to exploit resource consumption flaws. This particular vulnerability resides within the api.php file and specifically targets the checkcode functionality that handles captcha validation processes. The flaw manifests when remote attackers submit maliciously crafted requests containing excessively large values for font_size, height, and width parameters, which are typically used to configure visual elements of captcha images. This vulnerability represents a classic example of insufficient resource limitation, where the application fails to validate or constrain input parameters that directly influence memory and processing requirements during image generation operations.
The technical exploitation of this vulnerability occurs through a carefully constructed HTTP request to the api.php endpoint with the op=checkcode parameter. When PHPCMS processes these requests, it attempts to generate captcha images using the provided parameters without adequate bounds checking or resource allocation limits. The excessive font_size, height, and width values cause the system to allocate disproportionately large amounts of memory for image processing operations, leading to resource exhaustion that can ultimately result in system instability or complete service unavailability. This flaw operates at the application layer and can be leveraged by attackers to consume system resources such as memory and cpu cycles at an accelerated rate, effectively creating a denial of service condition that impacts legitimate users attempting to access the platform.
The operational impact of this vulnerability extends beyond simple service disruption to encompass potential cascading effects on system availability and performance. When exploited, the vulnerability can cause the web server or application container to exhaust available memory resources, leading to process termination, system slowdowns, or complete application crashes. This type of resource exhaustion attack aligns with the attack pattern described in the MITRE ATT&CK framework under the "Resource Exhaustion" technique, where adversaries consume system resources to deny service to legitimate users. The vulnerability is particularly concerning because it requires minimal privileges to exploit and can be executed through standard web browser interactions, making it accessible to attackers with basic technical knowledge. Organizations running affected PHPCMS installations face significant risk of operational disruption, potential revenue loss, and reputational damage if this vulnerability remains unpatched.
Mitigation strategies for CVE-2018-14940 should focus on implementing proper input validation and resource limitation mechanisms within the PHPCMS application. The most effective approach involves modifying the api.php script to enforce strict bounds checking on all image generation parameters, particularly font_size, height, and width values. Implementing maximum value limits for these parameters prevents attackers from submitting excessively large values that would trigger resource exhaustion. Additionally, organizations should consider implementing rate limiting mechanisms to restrict the number of captcha requests that can be processed within a given time period, further reducing the potential impact of exploitation attempts. The vulnerability also highlights the importance of following secure coding practices as outlined in CWE-1321, which addresses insufficient resource limit checks in applications. System administrators should also deploy intrusion detection systems that can identify unusual patterns of captcha requests and alert security teams to potential exploitation attempts. Regular security updates and patches should be applied promptly to address this vulnerability, as PHPCMS has released versions that contain fixes for this specific issue.