In case of sending request with empty host field:

GET /redirect-https-test/ HTTP/1.1 [ host: , Connection: ....])

server return response 500 Internal Server Error instead 400 Bad Request which is against HTTP/1.1 spec RFC (5.4 Host section)

Additional info

There is different behavior which depends on require-host-http11 option

When require-host-http11 options set as TRUE, it does works as expected:

curl -I --header 'Host: ' http://127.0.0.1:8080
HTTP/1.1 400 Bad Request
Content-Length: 0
Connection: close

curl -I --header 'Host: ' http://127.0.0.1:8080/jbeap8946_dep/index.html
HTTP/1.1 400 Bad Request
Content-Length: 0
Connection: close

With require-host-http11 options set as FALSE:

Simple url. Expected behavior.

curl -I --header 'Host: ' http://127.0.0.1:8080/
HTTP/1.1 200 OK
Connection: keep-alive
Last-Modified: Sat, 29 Jul 2017 08:38:24 GMT
X-Powered-By: Undertow/1
Server: JBoss-EAP/7
Content-Length: 1517
Content-Type: text/html
Accept-Ranges: bytes
Date: Mon, 14 Aug 2017 06:27:04 GMT

But in case with redirect(HTTP is redirected to HTTPS)

curl -I --header 'Host: ' http://127.0.0.1:8080/jbeap8946_dep/index.html
HTTP/1.1 500 Internal Server Error
Connection: keep-alive
X-Powered-By: Undertow/1
Server: JBoss-EAP/7
Content-Length: 86
Content-Type: text/html
Date: Mon, 14 Aug 2017 06:26:42 GMT

there is exception during parsing URI.

08:26:42,202 ERROR [io.undertow.request] (default task-1) UT005001: An exception occurred processing the request: java.net.URISyntaxException: Expected hostname at index 8: https://:8443/jbeap8946_dep/index.html
at java.net.URI$Parser.fail(URI.java:2848)
at java.net.URI$Parser.failExpecting(URI.java:2854)
at java.net.URI$Parser.parseHostname(URI.java:3390)
at java.net.URI$Parser.parseServer(URI.java:3236)
at java.net.URI$Parser.parseAuthority(URI.java:3155)
at java.net.URI$Parser.parseHierarchical(URI.java:3097)
at java.net.URI$Parser.parse(URI.java:3053)
at java.net.URI.<init>(URI.java:673)
at io.undertow.security.handlers.SinglePortConfidentialityHandler.getRedirectURI(SinglePortConfidentialityHandler.java:64)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.getRedirectURI(ServletConfidentialityConstraintHandler.java:83)
at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:49)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)
at io.undertow.servlet.handlers.security.ServletSecurityConstraintHandler.handleRequest(ServletSecurityConstraintHandler.java:59)
at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60)
at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77)
at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50)
at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.deployment.GlobalRequestControllerHandler.handleRequest(GlobalRequestControllerHandler.java:68)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:292)
at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:138)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:135)
at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48)
at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
at org.wildfly.extension.undertow.security.SecurityContextThreadSetupAction.lambda$create$0(SecurityContextThreadSetupAction.java:105)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1506)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1506)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1506)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1506)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:272)
at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:104)
at io.undertow.server.Connectors.executeRootHandler(Connectors.java:332)
at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:812)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)