diff --git a/bazel/external/http_parser/http_parser.c b/bazel/external/http_parser/http_parser.c
index 07c93dce1a697..5d01db6d52aa1 100644
--- a/bazel/external/http_parser/http_parser.c
+++ b/bazel/external/http_parser/http_parser.c
@@ -1079,6 +1079,10 @@ size_t http_parser_execute (http_parser *parser,
             break;
           case CR:
           case LF:
+            if (parser->method == HTTP_CONNECT) {
+              SET_ERRNO(HPE_INVALID_VERSION);
+              goto error;
+            }
             parser->http_major = 0;
             parser->http_minor = 9;
             UPDATE_STATE((ch == CR) ?
diff --git a/changelogs/current/bug_fixes/http__versionless-connect-response.rst b/changelogs/current/bug_fixes/http__versionless-connect-response.rst
new file mode 100644
index 0000000000000..d25a05df969a0
--- /dev/null
+++ b/changelogs/current/bug_fixes/http__versionless-connect-response.rst
@@ -0,0 +1,3 @@
+Fixed a bug where malformed CONNECT request lines without an authority could cause the legacy
+HTTP/1 parser to encode a ``400 Bad Request`` response using HTTP/1.0. Envoy now rejects these
+requests without downgrading the response protocol.
diff --git a/test/common/http/http1/BUILD b/test/common/http/http1/BUILD
index 21e417fda1913..e98b3573cc4a9 100644
--- a/test/common/http/http1/BUILD
+++ b/test/common/http/http1/BUILD
@@ -32,6 +32,7 @@ envoy_cc_test(
         "//source/common/http:exception_lib",
         "//source/common/http:header_map_lib",
         "//source/common/http/http1:codec_lib",
+        "//source/common/http/http1:legacy_parser_lib",
         "//source/extensions/http/header_validators/envoy_default:http1_header_validator",
         "//test/common/memory:memory_test_utility_lib",
         "//test/common/stats:stat_test_utility_lib",
diff --git a/test/common/http/http1/codec_impl_test.cc b/test/common/http/http1/codec_impl_test.cc
index 17c3a4c8160b3..28150129e5de7 100644
--- a/test/common/http/http1/codec_impl_test.cc
+++ b/test/common/http/http1/codec_impl_test.cc
@@ -14,6 +14,7 @@
 #include "source/common/http/exception.h"
 #include "source/common/http/header_map_impl.h"
 #include "source/common/http/http1/codec_impl.h"
+#include "source/common/http/http1/legacy_parser_impl.h"
 #include "source/common/runtime/runtime_impl.h"
 #include "source/extensions/http/header_validators/envoy_default/http1_header_validator.h"
 
@@ -169,6 +170,15 @@ class Http1CodecTestBase : public ::testing::Test {
   NiceMock<Server::MockOverloadManager> overload_manager_;
 };
 
+class LegacyParserServerConnectionImpl : public Http1::ServerConnectionImpl {
+public:
+  using Http1::ServerConnectionImpl::ServerConnectionImpl;
+
+  void useLegacyParser() {
+    parser_ = std::make_unique<Http1::LegacyHttpParserImpl>(Http1::MessageType::Request, this);
+  }
+};
+
 class Http1ServerConnectionImplTest : public Http1CodecTestBase {
 public:
   void initialize() {
@@ -947,6 +957,43 @@ TEST_F(Http1ServerConnectionImplTest, Http10) {
   EXPECT_EQ(Protocol::Http10, codec_->protocol());
 }
 
+TEST_F(Http1ServerConnectionImplTest, ConnectWithoutAuthorityUsesHttp11WithLegacyParser) {
+  codec_settings_.accept_http_10_ = true;
+  auto legacy_codec = std::make_unique<LegacyParserServerConnectionImpl>(
+      connection_, http1CodecStats(), callbacks_, codec_settings_, max_request_headers_kb_,
+      max_request_headers_count_, headers_with_underscores_action_, overload_manager_);
+  legacy_codec->useLegacyParser();
+  codec_ = std::move(legacy_codec);
+
+  MockRequestDecoder decoder;
+  setupRequestDecoderMock(decoder);
+  Http::ResponseEncoder* response_encoder = nullptr;
+  EXPECT_CALL(callbacks_, newStream(_, _))
+      .WillOnce(Invoke([&](ResponseEncoder& encoder, bool) -> RequestDecoder& {
+        response_encoder = &encoder;
+        return decoder;
+      }));
+  EXPECT_CALL(decoder,
+              sendLocalReply(Http::Code::BadRequest, "Bad Request", _, _, "http1.codec_error"));
+
+  Buffer::OwnedImpl buffer("CONNECT  HTTP/1.1\r\n"
+                           "Host: foo.bar.com:80\r\n"
+                           "foo: bar\r\n"
+                           "baz: fuz\r\n"
+                           "\r\n");
+  const auto status = codec_->dispatch(buffer);
+
+  EXPECT_TRUE(isCodecProtocolError(status));
+  EXPECT_EQ("http/1.1 protocol error: HPE_INVALID_VERSION", status.message());
+  EXPECT_EQ(Protocol::Http11, codec_->protocol());
+  EXPECT_EQ("http1.codec_error", response_encoder->getStream().responseDetails());
+
+  std::string output;
+  ON_CALL(connection_, write(_, _)).WillByDefault(AddBufferToString(&output));
+  response_encoder->encodeHeaders(TestResponseHeaderMapImpl{{":status", "400"}}, true);
+  EXPECT_EQ("HTTP/1.1 400 Bad Request\r\ncontent-length: 0\r\n\r\n", output);
+}
+
 TEST_F(Http1ServerConnectionImplTest, Http10HostAdded) {
   codec_settings_.accept_http_10_ = true;
   codec_settings_.default_host_for_http_10_ = "example.com";