PYSEC-2026-1055

Twisted vulnerable to NameVirtualHost Host header injection

When the host header does not match a configured host, `twisted.web.vhost.NameVirtualHost` will return a `NoResource` resource which renders the Host header unescaped into the 404 response allowing HTML and script injection. Example configuration: ```python from twisted.web.server import Site from twisted.web.vhost import NameVirtualHost from twisted.internet import reactor resource = NameVirtualHost() site = Site(resource) reactor.listenTCP(8080, site) reactor.run() ``` Output: ``` ❯ curl -H"Host:<h1>HELLO THERE</h1>" http://localhost:8080/ <html> <head><title>404 - No Such Resource</title></head> <body> <h1>No Such Resource</h1> <p>host b'<h1>hello there</h1>' not in vhost map</p> </body> </html> ``` This vulnerability was introduced in f49041bb67792506d85aeda9cf6157e92f8048f4 and first appeared in the 0.9.4 release.