The vulnerability was used to compromise vulnerable websites for 16000 (sometimes - 20000) times per day.
Vulnerable component: Joomla!
CVSSv3 score: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:H/RL:O/RC:C
CWE-ID: CWE-94 - Improper Control of Generation of Code ('Code Injection')
The vulnerability allows a remote attacker to execute arbitrary PHP code on the target system.
The vulnerability exists due to insufficient filtration of HTTP User-Agent header and filter-search HTTP POST parameter before storing them into database. A remote unauthenticated attacker can permanently inject and execute arbitrary PHP code on the target system with privileges of the web server.
Successful exploitation of this vulnerability will allow a remote attacker to gain complete control over the vulnerable web application and execute arbitrary PHP code on the target system.
Note: this is a zero-day vulnerability and it is being exploited in the wild.