CVE-2026-54611 in icms2
Summary
by MITRE • 09/08/2026
InstantCMS is a free and open source content management system. Versions prior to 2.18.2 have a Remote Code Execution (RCE) issue that allows remote authenticated attackers to execute any PHP code via the component installer. It is possible to upload a malicious component into the server, however, it won't be installed, but upload files will be executed. Normally all php files in upload folder are not executed, however, by uploading custom .htaccess it becomes possible. Version 2.18.2 contains a fix.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified in InstantCMS versions prior to 2.18.2 represents a critical Remote Code Execution flaw that stems from improper handling of file uploads within the application's component installer module. Although the system is designed to prevent the installation and execution of arbitrary PHP code through this interface, an authenticated attacker can bypass these safeguards by exploiting specific server configuration behaviors related to Apache web servers. The core technical issue lies in the interaction between the uploaded files and the .htaccess directive mechanism. Typically, web servers are configured with security policies that prohibit the direct execution of scripts within upload directories to mitigate risks associated with user-uploaded content. However, InstantCMS allows authenticated users to upload arbitrary files through its component installer interface without sufficient validation or sanitization of these uploads regarding their final destination and intended use.
By uploading a specifically crafted .htaccess file alongside malicious PHP code into the designated upload directory, an attacker can alter the server's processing rules for that specific folder. The uploaded .htaccess file contains directives that instruct Apache to treat files with certain extensions as executable scripts or to disable existing security restrictions on script execution within that path. Once this configuration change is applied by the web server, any PHP files previously uploaded to that directory become immediately executable. This effectively neutralizes the default safety mechanism of the upload folder, allowing the attacker to trigger arbitrary code execution simply by accessing the URL of the malicious PHP file via a browser or automated tool.
This vulnerability aligns with CWE-94 Improper Control of Generation of Code and is classified under ATT&CK technique T1505 Server Component Installation when considering the initial vector through the component installer, followed by T1059 Command and Scripting Interpreter for the actual execution phase. The impact of this flaw is severe as it grants remote authenticated attackers full control over the underlying server environment. With PHP code execution capabilities, an attacker can read sensitive configuration files containing database credentials, exfiltrate user data, create backdoors for persistent access, or pivot to attack other systems within the internal network depending on the privileges under which the web server process operates. The requirement for authentication does not significantly mitigate the risk in environments where administrative accounts may be compromised through credential stuffing, phishing, or other lateral movement techniques.
The remediation strategy is straightforward and was addressed in InstantCMS version 2.18.2 by implementing stricter validation on uploaded files to prevent the upload of configuration files such as .htaccess that could modify server behavior. Organizations running affected versions must upgrade immediately to the patched release. In addition to upgrading, administrators should ensure that web server configurations enforce strict MIME type checking and restrict file uploads to non-executable extensions where possible. Implementing a Web Application Firewall with rulesets capable of detecting attempts to upload .htaccess files or modify Apache configuration directives can provide an additional layer of defense during the transition period. Regular security audits and penetration testing should be conducted to verify that no other similar misconfigurations exist within the application's file handling modules, ensuring comprehensive protection against code injection attacks.