Allow overriding the host header if doesn't match the absolute-form host

src/Servers/Kestrel/Core/src/Internal/Http/Http1Connection.cs

@@ -603,7 +603,22 @@ private void ValidateNonOriginHostHeader(string hostText)
                     if (!_absoluteRequestTarget.IsDefaultPort
                         || hostText != _absoluteRequestTarget.Authority + ":" + _absoluteRequestTarget.Port.ToString(CultureInfo.InvariantCulture))
                     {
-                        KestrelBadHttpRequestException.Throw(RequestRejectionReason.InvalidHostHeader, hostText);
+                        // Superseded by RFC 7230, but notable for back-compat.
+                        // https://datatracker.ietf.org/doc/html/rfc2616/#section-5.2
+                        //    1. If Request-URI is an absoluteURI, the host is part of the
+                        // Request-URI. Any Host header field value in the request MUST be
+                        // ignored.
+                        // We don't want to leave the invalid value for the app to accidentally consume,
+                        // replace it with the value from the request line.
+                        if (_context.ServiceContext.ServerOptions.EnableInsecureAbsoluteFormHostOverride)
+                        {
+                            hostText = _absoluteRequestTarget.Authority + ":" + _absoluteRequestTarget.Port.ToString(CultureInfo.InvariantCulture);
+                            HttpRequestHeaders.HeaderHost = hostText;
+                        }
+                        else
+                        {
+                            KestrelBadHttpRequestException.Throw(RequestRejectionReason.InvalidHostHeader, hostText);
+                        }
                     }
                 }
             }

src/Servers/Kestrel/Core/src/KestrelServerOptions.cs

@@ -36,6 +36,21 @@ public class KestrelServerOptions
 
         private Func<string, Encoding?> _responseHeaderEncodingSelector = DefaultHeaderEncodingSelector;
 
+        private bool? _enableInsecureAbsoluteFormHostOverride;
+        internal bool EnableInsecureAbsoluteFormHostOverride
+        {
+            get
+            {
+                if (!_enableInsecureAbsoluteFormHostOverride.HasValue)
+                {
+                    _enableInsecureAbsoluteFormHostOverride =
+                        AppContext.TryGetSwitch("Microsoft.AspNetCore.Server.Kestrel.EnableInsecureAbsoluteFormHostOverride", out var enabled) && enabled;
+                }
+                return _enableInsecureAbsoluteFormHostOverride.Value;
+            }
+            set => _enableInsecureAbsoluteFormHostOverride = value;
+        }
+
         // The following two lists configure the endpoints that Kestrel should listen to. If both lists are empty, the "urls" config setting (e.g. UseUrls) is used.
         internal List<ListenOptions> CodeBackedListenOptions { get; } = new List<ListenOptions>();
         internal List<ListenOptions> ConfigurationBackedListenOptions { get; } = new List<ListenOptions>();

src/Servers/Kestrel/test/InMemory.FunctionalTests/BadHttpRequestTests.cs

@@ -8,6 +8,7 @@
 using Microsoft.AspNetCore.Server.Kestrel.InMemory.FunctionalTests.TestTransport;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Primitives;
 using Moq;
 using Xunit;
 using BadHttpRequestException = Microsoft.AspNetCore.Http.BadHttpRequestException;
@@ -137,6 +138,30 @@ public Task BadRequestIfHostHeaderDoesNotMatchRequestTarget(string requestTarget
                 CoreStrings.FormatBadRequest_InvalidHostHeader_Detail(host.Trim()));
         }
 
+        [Fact]
+        public async Task CanOptOutOfBadRequestIfHostHeaderDoesNotMatchRequestTarget()
+        {
+            var receivedHost = StringValues.Empty;
+            await using var server = new TestServer(context =>
+            {
+                receivedHost = context.Request.Headers.Host;
+                return Task.CompletedTask;
+            }, new TestServiceContext(LoggerFactory)
+            {
+                ServerOptions = new KestrelServerOptions()
+                {
+                    EnableInsecureAbsoluteFormHostOverride = true,
+                }
+            });
+            using var client = server.CreateConnection();
+
+            await client.SendAll($"GET http://www.foo.com/api/data HTTP/1.1\r\nHost: www.foo.comConnection: keep-alive\r\n\r\n");
+
+            await client.Receive("HTTP/1.1 200 OK");
+
+            Assert.Equal("www.foo.com:80", receivedHost);
+        }
+
         [Fact]
         public Task BadRequestFor10BadHostHeaderFormat()
         {