CVE-2025-55824: Modstart CMS RCE Vulnerability

CVE-2025-55824 Overview

CVE-2025-55824 is an arbitrary file write vulnerability in ModStartCMS v9.5.0. The flaw allows unauthenticated network-based attackers to write malicious files to the server and execute commands, leading to disclosure of sensitive data. The weakness is classified under [CWE-77] Improper Neutralization of Special Elements used in a Command (Command Injection). ModStartCMS is a PHP-based content management framework used to build corporate and content-driven websites. Because the attack requires no authentication and no user interaction, attackers can target exposed instances directly over HTTP.

Critical Impact
Attackers can write arbitrary files to the ModStartCMS host and execute commands, exposing sensitive server-side data.

Affected Products

ModStartCMS 9.5.0
Vendor: modstart
Product identifier: modstart:mostartcms

Discovery Timeline

2025-09-02 - CVE-2025-55824 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-55824

Vulnerability Analysis

CVE-2025-55824 is an arbitrary file write issue that maps to [CWE-77], command injection. An attacker can send a crafted request to a vulnerable ModStartCMS v9.5.0 endpoint and cause the application to write attacker-controlled content to a server-side path. Once a malicious file, such as a PHP web shell, is dropped inside a web-accessible directory, the attacker can request it to execute arbitrary commands under the web server user. The resulting code execution enables reading configuration files, database credentials, and other sensitive data stored on the host.

The attack vector is network-based, does not require authentication, and does not require user interaction. Exploitation results in limited confidentiality and integrity impact within the application context but no direct availability impact, according to the published CVSS vector. The current EPSS probability is 0.235% (percentile 14.442), indicating low observed exploitation likelihood at this time.

Root Cause

The root cause is missing neutralization of user-supplied input that is used to construct file paths or shell commands during a file write operation. Because ModStartCMS accepts controllable parameters without sufficient validation, an attacker can traverse directories or inject payloads into the write routine.

Attack Vector

An unauthenticated attacker sends a crafted HTTP request to the vulnerable endpoint. The request supplies both the target file path and file contents. The server writes the payload to disk, after which the attacker retrieves the dropped file over HTTP to trigger execution. Technical exploitation details are described in the public GitHub Gist PoC Code.

Detection Methods for CVE-2025-55824

Indicators of Compromise

New or unexpected .php, .phtml, or .phar files appearing under ModStartCMS web-accessible directories such as public/, storage/, or uploads/.
HTTP POST requests to ModStartCMS endpoints containing path traversal sequences like ../ or absolute paths in parameter values.
Outbound network connections initiated by the PHP-FPM or web server process to unfamiliar external hosts shortly after suspicious write activity.

Detection Strategies

Monitor web server access logs for anomalous POST requests to ModStartCMS administrative or upload endpoints originating from unauthenticated sessions.
Deploy file integrity monitoring on the ModStartCMS document root to alert on creation or modification of executable script files.
Correlate web request logs with process execution telemetry to identify web server processes spawning shells such as sh, bash, or cmd.exe.

Monitoring Recommendations

Enable verbose PHP and web server logging, including request bodies where policy allows, to capture exploitation attempts.
Alert on child processes of the web server user executing reconnaissance commands such as whoami, id, uname, or cat /etc/passwd.
Baseline outbound traffic from the ModStartCMS host and alert on deviations that may indicate data exfiltration.

How to Mitigate CVE-2025-55824

Immediate Actions Required

Restrict network access to ModStartCMS v9.5.0 administrative interfaces using firewall rules or IP allowlists until a patched version is deployed.
Place the application behind a web application firewall configured to block path traversal patterns and suspicious file write parameters.
Audit the web root and upload directories for unauthorized script files and remove any that are not part of the legitimate deployment.
Rotate credentials, API keys, and database secrets stored on or accessible from the ModStartCMS host if compromise is suspected.

Patch Information

No vendor advisory or fixed release is listed in the enriched CVE data at time of writing. Administrators should monitor the ModStart project channels and upstream repository for an updated release beyond v9.5.0 and apply it as soon as it becomes available.

Workarounds

Configure the web server to deny execution of PHP files inside upload and storage directories using directives such as php_admin_flag engine off or equivalent location blocks.
Enforce least-privilege file system permissions so the web server user cannot write to directories that are also served as executable content.
Disable or restrict access to any ModStartCMS module known to accept file paths or file contents as user input until a patch is applied.

bash

# Example nginx configuration to block PHP execution in upload paths
location ~* ^/(uploads|storage|public/data)/.*\.(php|phtml|phar)$ {
    deny all;
    return 403;
}