Handle spaces after request line

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

@@ -16,14 +16,28 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
     public class HttpParser<TRequestHandler> : IHttpParser<TRequestHandler> where TRequestHandler : IHttpHeadersHandler, IHttpRequestLineHandler
     {
         private readonly bool _showErrorDetails;
+        private readonly bool _allowSpaceAfterRequestLine;
 
         public HttpParser() : this(showErrorDetails: true)
         {
         }
 
         public HttpParser(bool showErrorDetails)
+            : this (showErrorDetails, CheckAllowSpaceAfterRequestLine())
+        {
+        }
+
+        internal HttpParser(bool showErrorDetails, bool allowSpaceAfterRequestLine)
         {
             _showErrorDetails = showErrorDetails;
+            _allowSpaceAfterRequestLine = allowSpaceAfterRequestLine;
+        }
+
+        private static bool CheckAllowSpaceAfterRequestLine()
+        {
+            // This mitigation is temporary and 6.0 specific, we do not anticipate porting this feature to later versions.
+            AppContext.TryGetSwitch("Microsoft.AspNetCore.Server.Kestrel.AllowSpaceAfterRequestLine", out var allowSpaceAfterRequestLine);
+            return allowSpaceAfterRequestLine;
         }
 
         // byte types don't have a data type annotation so we pre-cast them; to avoid in-place casts
@@ -48,7 +62,25 @@ public bool ParseRequestLine(TRequestHandler handler, ref SequenceReader<byte> r
 
             if (reader.TryReadTo(out ReadOnlySpan<byte> requestLine, ByteLF, advancePastDelimiter: true))
             {
+                if (_allowSpaceAfterRequestLine)
+                {
+                    // Skip a space after the request line
+                    if (reader.TryPeek(out byte s) && s == ByteSpace)
+                    {
+                        reader.Advance(1);
+                    }
+                    // Don't parse the request line until we've started receiving headers.
+                    // Need to make sure we skipped an extra space if present.
+                    else if (reader.End)
+                    {
+                        // Reset state so we can try again.
+                        reader.Rewind(requestLine.Length + 1);
+                        return false;
+                    }
+                }
+
                 ParseRequestLine(handler, requestLine);
+
                 return true;
             }
 

src/Servers/Kestrel/Core/test/HttpParserTests.cs

@@ -52,6 +52,28 @@ public void ParsesRequestLine(
             Assert.True(buffer.Slice(examined).IsEmpty);
         }
 
+        [Fact]
+        public void ParsesRequestLineWithTrailingSpace()
+        {
+            var parser = CreateParser(_nullTrace, allowSpaceAfterRequestLine: true);
+            var buffer = new ReadOnlySequence<byte>(Encoding.ASCII.GetBytes("GET / HTTP/1.1\r\n "));
+            var requestHandler = new RequestHandler();
+
+            Assert.False(ParseRequestLine(parser, requestHandler, buffer.Slice(0, buffer.Length - 1), out var consumed, out var examined));
+            Assert.True(buffer.Slice(0, consumed).IsEmpty);
+            Assert.Equal(buffer.Length - 1, buffer.Slice(0, examined).Length);
+
+            Assert.True(ParseRequestLine(parser, requestHandler, buffer, out consumed, out examined));
+            Assert.True(buffer.Slice(consumed).IsEmpty);
+            Assert.True(buffer.Slice(examined).IsEmpty);
+
+            Assert.Equal(HttpMethods.Get, requestHandler.Method);
+            Assert.Equal("HTTP/1.1", requestHandler.Version);
+            Assert.Equal("/", requestHandler.RawTarget);
+            Assert.Equal("/", requestHandler.RawPath);
+            Assert.Equal("HTTP/1.1", requestHandler.Version);
+        }
+
         [Theory]
         [MemberData(nameof(RequestLineIncompleteData))]
         public void ParseRequestLineReturnsFalseWhenGivenIncompleteRequestLines(string requestLine)
@@ -536,7 +558,8 @@ private void VerifyRawHeaders(string rawHeaders, IEnumerable<string> expectedHea
             Assert.True(buffer.Slice(reader.Position).IsEmpty);
         }
 
-        private IHttpParser<RequestHandler> CreateParser(IKestrelTrace log) => new HttpParser<RequestHandler>(log.IsEnabled(LogLevel.Information));
+        private IHttpParser<RequestHandler> CreateParser(IKestrelTrace log, bool allowSpaceAfterRequestLine = false)
+            => new HttpParser<RequestHandler>(log.IsEnabled(LogLevel.Information), allowSpaceAfterRequestLine);
 
         public static IEnumerable<object[]> RequestLineValidData => HttpParsingData.RequestLineValidData;