In MyBB before 1.8.21, an attacker can abuse a default...

Vulnerability 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.

References

• https://blog.ripstech.com/2019/mybb-stored-xss-to-rce/
• https://blog.mybb.com/2019/06/10/mybb-1-8-21-released-security-maintenance-release/