CVE-2025-48940: MyBB Path Traversal Vulnerability

CVE-2025-48940 Overview

CVE-2025-48940 is a Local File Inclusion (LFI) vulnerability in MyBB, an open source forum software. The flaw exists in the upgrade component, which fails to properly validate user input. Attackers can supply a specially crafted parameter value to include arbitrary local files. Exploitation requires the installer to be unlocked (no install/lock file present) or authentication as a forum administrator. The vulnerability affects all MyBB versions prior to 1.8.39 and is tracked under [CWE-22] (Path Traversal). MyBB released version 1.8.39 to resolve the issue.

Critical Impact
Authenticated administrators or attackers accessing an unlocked installer can read sensitive local files and potentially achieve code execution through MyBB's upgrade script.

Affected Products

MyBB versions prior to 1.8.39
MyBB installations with an accessible install/index.php script
MyBB instances where install/lock is missing

Discovery Timeline

2025-06-02 - CVE-2025-48940 published to NVD
2025-07-02 - Last updated in NVD database

Technical Details for CVE-2025-48940

Vulnerability Analysis

The vulnerability resides in MyBB's upgrade component, which processes user-supplied parameters without proper validation or sanitization. An attacker can manipulate a parameter to traverse the filesystem and include arbitrary local files. The exploit path depends on the state of the MyBB installation. When install/lock is absent, the installer remains reachable and the upgrade script can be invoked. Authenticated forum administrators can also access the upgrade workflow directly. Successful exploitation can disclose sensitive files such as configuration data containing database credentials. Depending on the included file type, the issue can escalate to code execution within the MyBB process context. The flaw is classified as [CWE-22] (Improper Limitation of a Pathname to a Restricted Directory). EPSS data places the probability of exploitation activity at 0.505% with a 66.5 percentile ranking.

Root Cause

The upgrade component accepts a parameter value and passes it into a file inclusion routine without enforcing an allowlist or sanitizing path traversal sequences. This allows path components such as ../ or absolute path references to redirect the inclusion target outside the intended directory.

Attack Vector

The attack is delivered over the network through HTTP requests to MyBB's upgrade script. The attacker must either control a forum administrator account, target a forum that has not yet completed installation, or reach an instance where the installer was not locked after deployment.

// No verified public exploit code is available for CVE-2025-48940.
// Refer to the MyBB security advisory for technical details:
// https://github.com/mybb/mybb/security/advisories/GHSA-q4jv-xwjx-37cp

Detection Methods for CVE-2025-48940

Indicators of Compromise

HTTP requests to install/index.php or upgrade endpoints containing path traversal sequences such as ../ or encoded variants (%2e%2e%2f).
Unexpected access to install/index.php on a production forum after initial deployment.
Absence of the install/lock file on a fully deployed MyBB instance.
Web server logs showing upgrade script invocations from non-administrative IP ranges.

Detection Strategies

Inspect web server and application logs for requests targeting MyBB upgrade or install scripts with suspicious parameter values.
Deploy web application firewall rules to flag path traversal patterns directed at MyBB endpoints.
Run filesystem integrity checks to confirm the presence of install/lock on production deployments.
Correlate administrator authentication events with subsequent access to upgrade or install URLs.

Monitoring Recommendations

Alert on any HTTP 200 responses returned by install/index.php on production hosts.
Monitor outbound file reads originating from the PHP worker process serving MyBB.
Track changes to MyBB version metadata to confirm the deployed release is 1.8.39 or later.

How to Mitigate CVE-2025-48940

Immediate Actions Required

Upgrade MyBB to version 1.8.39 or later as documented in the MyBB 1.8.39 release notes.
Verify that the install/lock file exists on every production deployment to block re-installation flows.
Restrict access to install/ and upgrade.php paths at the web server or reverse proxy layer.
Review administrator accounts and rotate credentials for any account that may have been used to reach the upgrade workflow.

Patch Information

MyBB resolved the issue in version 1.8.39. The upstream fix is published as commit 6e6cfbd524d9101b51e1278ecf520479b64b0f00 and described in the GHSA-q4jv-xwjx-37cp advisory. Administrators should apply the official release rather than backporting the patch manually.

Workarounds

Block external access to install/ directories using web server configuration until the patch is applied.
Remove or rename the install/ directory on fully deployed forums where re-installation is not required.
Limit administrator panel access to trusted IP ranges through network ACLs.

bash

# Example nginx configuration to deny access to MyBB install directory
location ^~ /install/ {
    deny all;
    return 403;
}