This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in Acquia Mautic.
The vulnerability is due to improperly sanitized user metadata collected from tracking pixels. A remote, unauthenticated attacker could exploit this vulnerability by sending crafted data to the target server by interacting with a tracking pixel on an external site. Successful exploitation could result in execution of script code in the security context of target user's browser.
Acquia Mautic prior to 4.3.0
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.