CVE-2025-48940 is a high-severity vulnerability affecting MyBB, a widely used free and open-source forum software. The vulnerability is classified as CWE-22: Improper Limitation of a Pathname to a Restricted Directory, commonly known as a path traversal flaw. Specifically, in versions of MyBB prior to 1.8.39, the upgrade component fails to properly validate user input parameters. This flaw allows an attacker to perform Local File Inclusion (LFI) by manipulating a parameter value in the upgrade script. Exploiting this vulnerability requires that the installer be unlocked, meaning the 'install/lock' file is absent, and the upgrade script remains accessible. This condition can occur if the forum has not yet been installed, if the attacker can reinstall the forum via 'install/index.php', or if the attacker is authenticated as a forum administrator. Successful exploitation can lead to unauthorized reading of sensitive files on the server, potentially exposing configuration files, credentials, or other critical data. The vulnerability has a CVSS 3.1 base score of 7.2, reflecting high impact on confidentiality, integrity, and availability, with network attack vector, low attack complexity, and requiring high privileges but no user interaction. The issue was addressed in MyBB version 1.8.39, which includes proper input validation to prevent path traversal attacks in the upgrade component.

For European organizations using MyBB forum software, this vulnerability poses significant risks. If exploited, attackers could gain access to sensitive internal files, including configuration files containing database credentials or other secrets, leading to data breaches or further system compromise. The requirement for either the installer to be unlocked or administrative authentication limits the attack surface but does not eliminate risk, especially in environments where installation procedures are not properly finalized or where administrator credentials are compromised or weak. The potential impact includes unauthorized disclosure of confidential information, modification or deletion of files affecting forum integrity, and possible denial of service if critical files are manipulated. Given that many European organizations rely on forums for community engagement, customer support, or internal communication, exploitation could disrupt operations and damage reputation. Additionally, the vulnerability could be leveraged as a foothold for lateral movement within affected networks.

European organizations should immediately upgrade all MyBB installations to version 1.8.39 or later to remediate this vulnerability. Beyond patching, organizations must ensure that the installation process is properly completed and that the 'install/lock' file is present to prevent unauthorized access to installation or upgrade scripts. Access controls should be tightened to restrict access to the 'install' directory and upgrade scripts, ideally limiting it to trusted administrators only. Regular audits should verify that no installation or upgrade scripts remain accessible on production systems. Additionally, strong authentication mechanisms for forum administrators should be enforced, including multi-factor authentication where possible, to reduce the risk of credential compromise. Monitoring and logging access to installation and upgrade components can help detect attempted exploitation. Finally, organizations should conduct security reviews of their web applications to identify and remediate similar path traversal or file inclusion vulnerabilities.