This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in Flarum Flarum Core.
The vulnerability is due to incorrect sanitization of HTML Markup in Flarum's translation library resulting in both Stored and Reflected Cross-Site Scripting vectors. A remote attacker could exploit this vulnerability by sending a crafted request to the target system or enticing a user to click a crafted URL. A successful attack may result in execution of script code in the security context of the browser of any user visiting the affected pages.
Flarum Flarum Core 1.0.0
Flarum Flarum Core 1.0.1
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected application.
Apply the most recent upgrade or patch from the vendor.