Description
In MyBB before 1.8.21, an attacker can abuse a default behavior of MySQL on many systems (that leads to truncation of strings that are too long for a database column) to create a PHP shell in the cache directory of a targeted forum via a crafted XML import, as demonstrated by truncation of aaaaaaaaaaaaaaaaaaaaaaaaaa.php.css to aaaaaaaaaaaaaaaaaaaaaaaaaa.php with a 30-character limit, aka theme import stylesheet name RCE.
Remediation
References
Related Vulnerabilities
PHP Exposure of Sensitive Information to an Unauthorized Actor Vulnerability (CVE-2010-2100)
MySQL CVE-2012-0075 Vulnerability (CVE-2012-0075)
WordPress Plugin Apptivo Business Site CRM Multiple Cross-Site Scripting Vulnerabilities (1.2.9)
MyBB Exposure of Sensitive Information to an Unauthorized Actor Vulnerability (CVE-2019-3579)