Malicious HTML content could be injected into the page pretix shows when redirection to an untrusted page occurs. Since this page has a Content-Security-Policy, this can mainly be used for phishing purposes.
Exploitability
AV:NAC:HAT:NPR:LUI:PVulnerable System
VC:LVI:LVA:LSubsequent System
SC:LSI:LSA:L2.1/CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:P/VC:L/VI:L/VA:L/SC:L/SI:L/SA:LOther