A host header injection vulnerability in picocms/Pico through 2.1.4 allows an unauthenticated remote attacker to control the origin of JavaScript and CSS assets loaded by the default theme. When base_url is unset (the default), Pico::getBaseUrl in lib/Pico.php builds the base URL from unvalidated Host, X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Port request headers.