Executive Summary

CVE-2023-53979 is a chained vulnerability in MyBB version 1.8.32 that allows authenticated administrators to bypass avatar upload restrictions and execute arbitrary code remotely. The attacker first modifies the avatar upload path setting to a directory not properly blacklisted (./inc), enabling them to upload a malicious avatar file that is actually a PNG image embedded with PHP backdoor code. Then, by exploiting a Local File Inclusion (LFI) vulnerability in the language configuration editing interface, the attacker includes and executes the uploaded malicious file, resulting in remote code execution on the server. [1, 3, 4]

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can have severe impacts including allowing an authenticated administrator to execute arbitrary system commands on the server hosting the MyBB forum. This can lead to full compromise of the server, unauthorized access to sensitive data, modification or deletion of data, disruption of service, and potentially using the server as a pivot point for further attacks within the network. [1, 3, 4]

Useful 0 Not Useful 0

Detection Guidance

Detection involves verifying if the MyBB 1.8.32 instance has been configured with a modified avatar upload path (e.g., './inc') and if suspicious avatar files containing embedded PHP code exist in the upload directory. Additionally, monitoring HTTP POST requests to the admin language edit interface with parameters that include avatar files (e.g., file=avatar_1.png and editwith=..) can indicate exploitation attempts. Commands to check the upload path setting in the database or configuration files and to scan for suspicious avatar files include: 1) Query the database or configuration file for the 'avataruploadpath' setting to see if it is set to './inc' or other unusual paths. 2) List files in the upload directory (e.g., './inc') and inspect avatar files for embedded PHP code, for example using 'grep -a "<?php" ./inc/avatar_*.png'. 3) Monitor web server logs for POST requests to '/admin/index.php?module=config-languages&action=edit' with suspicious parameters. 4) Use web application firewall or IDS rules to detect attempts to exploit the LFI via directory traversal in language editing parameters. Note: Specific commands depend on the environment and access level. [1, 4]

Useful 0 Not Useful 0

Mitigation Strategies

Immediate mitigation steps include: 1) Restrict administrative access to the MyBB Admin Control Panel to trusted users only, as exploitation requires authenticated admin privileges. 2) Review and reset the 'avataruploadpath' setting to a safe directory that is properly validated and not writable by the web server or accessible for file inclusion. 3) Remove any suspicious avatar files from the upload directory, especially those in './inc' or other non-standard locations. 4) Apply any available patches or updates from MyBB that address this vulnerability or upgrade to a version where this issue is fixed. 5) Monitor and audit admin activities and web server logs for signs of exploitation attempts. 6) Consider implementing web application firewall rules to block attempts to exploit the LFI and file upload bypass. 7) If possible, disable or restrict the language editing interface to prevent exploitation via file inclusion. These steps help prevent attackers from uploading malicious files and executing arbitrary code. [1, 3, 4]

Useful 0 Not Useful 0

Compliance Impact

The provided resources do not contain information regarding the impact of CVE-2023-53979 on compliance with common standards and regulations such as GDPR or HIPAA.

Useful 0 Not Useful 0