Attack Chain

1. Initial Access: An attacker obtains valid administrator credentials for the MantisBT web UI, typically through phishing, credential stuffing, or other means.
2. Access Configuration Interface: The attacker logs into the MantisBT web UI and navigates to the "Manage Configuration" page (adm_config_set.php).
3. Craft Malicious Payload: The attacker prepares a PHP code payload that defines a malicious class, designed to exploit PHP's class hoisting mechanism.
4. Inject Payload: The attacker submits a POST request to adm_config_set.php, setting a configuration value to the malicious PHP class definition, ensuring it is treated as a non-string type.
5. Server-Side Processing: MantisBT's ConfigParser -> Tokenizer component processes the input, calling eval() on the provided configuration value.
6. Class Hoisting Bypass: Despite the return; prefix in the eval() call, PHP hoists the attacker-defined class at compile time.
7. Class Hijacking and Execution: The hoisted malicious class can then hijack a legitimate class that MantisBT subsequently attempts to load via its autoloader. This allows the attacker to execute arbitrary PHP code as the underlying web server user (e.g., www-data).
8. Post-Exploitation: The attacker gains full control over the MantisBT installation, potentially leading to data exfiltration, defacement, or further lateral movement within the network.

Impact

Successful exploitation of CVE-2026-49273 results in remote code execution as the web server user, which typically has privileges to read, write, and execute files within the web root and associated directories. This could lead to a complete compromise of the MantisBT application, including unauthorized access to project data, user credentials, and sensitive bug reports. Attackers could also use this access as a foothold to compromise the underlying server or other systems within the network. While the vulnerability requires authenticated administrator access, the impact is severe due to the potential for full system compromise.

Recommendation

• Patch CVE-2026-49273: Immediately update all MantisBT installations to version 2.28.4 or later by applying the patch from https://github.com/mantisbt/mantisbt/commit/78c0af63d1fe0118004744cab21ca3bf2cea0f5c to mitigate CVE-2026-49273.
• Monitor Web Server Logs: Deploy the provided Sigma rule to detect suspicious POST requests to adm_config_set.php containing PHP class or function definitions. Ensure web server logs capture cs-uri-stem and cs-uri-query for webserver log sources.
• Review Administrator Activity: Regularly audit logs for adm_config_set.php to identify unusual configuration changes, especially from unexpected IP addresses or at unusual times.