Introduce KestrelServerOptions.AllowUnsafeHostHeaderOverride

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

@@ -626,7 +626,15 @@ private void ValidateNonOriginHostHeader(string hostText)
                 if (!_absoluteRequestTarget.IsDefaultPort
                     || hostText != _absoluteRequestTarget.Authority + ":" + _absoluteRequestTarget.Port.ToString(CultureInfo.InvariantCulture))
                 {
-                    KestrelBadHttpRequestException.Throw(RequestRejectionReason.InvalidHostHeader, hostText);
+                    if (_context.ServiceContext.ServerOptions.AllowHostHeaderOverride)
+                    {
+                        hostText = _absoluteRequestTarget.Authority + ":" + _absoluteRequestTarget.Port.ToString(CultureInfo.InvariantCulture);
+                        HttpRequestHeaders.HeaderHost = hostText;
+                    }
+                    else
+                    {
+                        KestrelBadHttpRequestException.Throw(RequestRejectionReason.InvalidHostHeader, hostText);
+                    }
                 }
             }
         }

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

@@ -35,6 +35,29 @@ public class KestrelServerOptions
 
     private Func<string, Encoding?> _responseHeaderEncodingSelector = DefaultHeaderEncodingSelector;
 
+    /// <summary>
+    /// In HTTP/1.x, when a request target is in absolute-form (see RFC 9112 Section 3.2.2),
+    /// for example
+    /// <code>
+    /// GET http://www.example.com/path/to/index.html HTTP/1.1
+    /// </code>
+    /// the Host header is redundant.  In fact, the RFC says
+    ///
+    ///   When an origin server receives a request with an absolute-form of request-target,
+    ///   the origin server MUST ignore the received Host header field (if any) and instead
+    ///   use the host information of the request-target.
+    ///
+    /// However, it is still sensible to check whether the request target and Host header match
+    /// because a mismatch might indicate, for example, a spoofing attempt.  Setting this property
+    /// to true bypasses that check and unconditionally overwrites the Host header with the value
+    /// from the request target.
+    /// </summary>
+    /// <remarks>
+    /// This option does not apply to HTTP/2 or HTTP/3.
+    /// </remarks>
+    /// <seealso href="https://datatracker.ietf.org/doc/html/rfc9112#section-3.2.2-8"/>
+    public bool AllowHostHeaderOverride { get; set; }
+
     // 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/Core/src/PublicAPI.Unshipped.txt

@@ -1,6 +1,8 @@
 #nullable enable
 Microsoft.AspNetCore.Server.Kestrel.Core.Features.ISslStreamFeature
 Microsoft.AspNetCore.Server.Kestrel.Core.Features.ISslStreamFeature.SslStream.get -> System.Net.Security.SslStream!
+Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerOptions.AllowHostHeaderOverride.get -> bool
+Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerOptions.AllowHostHeaderOverride.set -> void
 Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerOptions.ListenNamedPipe(string! pipeName) -> void
 Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerOptions.ListenNamedPipe(string! pipeName, System.Action<Microsoft.AspNetCore.Server.Kestrel.Core.ListenOptions!>! configure) -> void
 Microsoft.AspNetCore.Server.Kestrel.Core.ListenOptions.PipeName.get -> string?

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

@@ -11,6 +11,7 @@
 using Microsoft.AspNetCore.Testing;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Primitives;
 using Moq;
 using Xunit;
 using BadHttpRequestException = Microsoft.AspNetCore.Http.BadHttpRequestException;
@@ -140,6 +141,32 @@ public Task BadRequestIfHostHeaderDoesNotMatchRequestTarget(string requestTarget
             CoreStrings.FormatBadRequest_InvalidHostHeader_Detail(host.Trim()));
     }
 
+    [Theory]
+    [InlineData("Host: www.foo.comConnection: keep-alive")] // Corrupted - missing line-break
+    [InlineData("Host: www.notfoo.com")] // Syntactically correct but not matching
+    public async Task CanOptOutOfBadRequestIfHostHeaderDoesNotMatchRequestTarget(string hostHeader)
+    {
+        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()
+            {
+                AllowHostHeaderOverride = true,
+            }
+        });
+        using var client = server.CreateConnection();
+
+        await client.SendAll($"GET http://www.foo.com/api/data HTTP/1.1\r\n{hostHeader}\r\n\r\n");
+
+        await client.Receive("HTTP/1.1 200 OK");
+
+        Assert.Equal("www.foo.com:80", receivedHost);
+    }
+
     [Fact]
     public Task BadRequestFor10BadHostHeaderFormat()
     {