CVE-2026-28133 in Filr Plugin
Summary
by MITRE • 03/05/2026
Unrestricted Upload of File with Dangerous Type vulnerability in WP Chill Filr filr-protection allows Upload a Web Shell to a Web Server.This issue affects Filr: from n/a through <= 1.2.12.
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Analysis
by VulDB Data Team • 03/11/2026
The vulnerability CVE-2026-28133 represents a critical security flaw in the WP Chill Filr filr-protection plugin that enables unauthorized file uploads with potentially malicious content. This issue exists within the filr plugin version range from an unspecified starting point through version 1.2.12, creating a persistent risk for WordPress installations that utilize this particular plugin. The vulnerability specifically allows attackers to upload files with dangerous types, including web shells, which can provide remote code execution capabilities on the affected web server. The unrestricted nature of this upload functionality means that proper validation and sanitization measures are either missing or inadequate, creating a direct pathway for malicious actors to compromise the target system.
The technical implementation of this vulnerability stems from insufficient input validation and file type checking mechanisms within the plugin's upload functionality. Attackers can exploit this weakness by crafting malicious files with extensions that bypass the intended security restrictions, typically through the use of file name manipulation or by leveraging the application's trust in certain file types. The vulnerability directly maps to CWE-434, which describes the weakness of unrestricted upload of dangerous file types, and represents a classic example of insecure file upload handling that has been consistently identified as a critical threat in web application security assessments. The attack vector involves sending specially crafted HTTP requests to the vulnerable plugin's file upload endpoint, where the application fails to properly validate the file content and extension against a comprehensive whitelist of allowed types.
The operational impact of this vulnerability is severe and multifaceted, as it provides attackers with persistent access to the compromised web server through the uploaded web shell. Once successful, the attacker gains the ability to execute arbitrary commands on the server, potentially leading to complete system compromise, data exfiltration, or the establishment of a persistent backdoor. This vulnerability affects the integrity and confidentiality of the entire WordPress installation, as the web shell can be used to escalate privileges, modify files, or establish further footholds within the network. The attack follows patterns consistent with the ATT&CK framework's T1190 (Exploit Public-Facing Application) and T1059 (Command and Scripting Interpreter) techniques, where attackers leverage application vulnerabilities to execute malicious code and maintain persistent access. Organizations running affected versions of the filr plugin face significant risk of data breaches, service disruption, and potential regulatory compliance violations.
Mitigation strategies for CVE-2026-28133 must address both immediate remediation and long-term security improvements. The primary recommendation is to update the filr plugin to a version that has been patched to address the unrestricted file upload vulnerability, ensuring that proper file type validation and content checking mechanisms are implemented. Organizations should implement strict file upload restrictions by maintaining a comprehensive whitelist of allowed file extensions and MIME types, while also performing content inspection of uploaded files to detect potentially malicious payloads. Network-level protections such as web application firewalls and intrusion detection systems should be configured to monitor and block suspicious file upload patterns. Additionally, the principle of least privilege should be applied by restricting upload capabilities to only those users who require them for legitimate business purposes, and by implementing proper file storage segregation that prevents uploaded files from being directly executable by the web server. Regular security audits and penetration testing should be conducted to identify similar vulnerabilities in other plugins and themes, while automated scanning tools should be employed to continuously monitor for the presence of vulnerable components within the web application infrastructure.