net/http: reject requests with invalid Content-Length headers

According to RFC 9110 and RFC 9112, invalid "Content-Length" headers
might involve request smuggling or response splitting, which could
also cause security failures. Currently, `net/http` ignores all
"Content-Length" headers when there is a "Transfer-Encoding" header and
forward the message anyway while other mainstream HTTP implementations
such as Apache Tomcat, Nginx, HAProxy, Node.js, Deno, Tornado, etc. reject
invalid Content-Length headers regardless of the presence of a
"Transfer-Encoding" header and only forward chunked-encoding messages
with either valid "Content-Length" headers or no "Content-Length" headers.

Fixes #65505

Change-Id: I73af2ee0785137e56c7546a4cce4a5c5c348dbc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/561075
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/readrequest_test.go

@@ -207,6 +207,22 @@ var reqTests = []reqTest{
 		noError,
 	},
 
+	// Tests chunked body and an invalid Content-Length.
+	{
+		"POST / HTTP/1.1\r\n" +
+			"Host: foo.com\r\n" +
+			"Transfer-Encoding: chunked\r\n" +
+			"Content-Length: notdigits\r\n\r\n" + // raise an error
+			"3\r\nfoo\r\n" +
+			"3\r\nbar\r\n" +
+			"0\r\n" +
+			"\r\n",
+		nil,
+		noBodyStr,
+		noTrailer,
+		`bad Content-Length "notdigits"`,
+	},
+
 	// CONNECT request with domain name:
 	{
 		"CONNECT www.google.com:443 HTTP/1.1\r\n\r\n",

src/net/http/serve_test.go

@@ -4819,6 +4819,12 @@ func TestServerValidatesHeaders(t *testing.T) {
 		// See RFC 7230, Section 3.2.
 		{": empty key\r\n", 400},
 
+		// Requests with invalid Content-Length headers should be rejected
+		// regardless of the presence of a Transfer-Encoding header.
+		// Check out RFC 9110, Section 8.6 and RFC 9112, Section 6.3.3.
+		{"Content-Length: notdigits\r\n", 400},
+		{"Content-Length: notdigits\r\nTransfer-Encoding: chunked\r\n\r\n0\r\n\r\n", 400},
+
 		{"foo: foo foo\r\n", 200},    // LWS space is okay
 		{"foo: foo\tfoo\r\n", 200},   // LWS tab is okay
 		{"foo: foo\x00foo\r\n", 400}, // CTL 0x00 in value is bad

src/net/http/transfer.go

@@ -650,19 +650,6 @@ func (t *transferReader) parseTransferEncoding() error {
 		return &unsupportedTEError{fmt.Sprintf("unsupported transfer encoding: %q", raw[0])}
 	}
 
-	// RFC 7230 3.3.2 says "A sender MUST NOT send a Content-Length header field
-	// in any message that contains a Transfer-Encoding header field."
-	//
-	// but also: "If a message is received with both a Transfer-Encoding and a
-	// Content-Length header field, the Transfer-Encoding overrides the
-	// Content-Length. Such a message might indicate an attempt to perform
-	// request smuggling (Section 9.5) or response splitting (Section 9.4) and
-	// ought to be handled as an error. A sender MUST remove the received
-	// Content-Length field prior to forwarding such a message downstream."
-	//
-	// Reportedly, these appear in the wild.
-	delete(t.Header, "Content-Length")
-
 	t.Chunked = true
 	return nil
 }
@@ -670,7 +657,7 @@ func (t *transferReader) parseTransferEncoding() error {
 // Determine the expected body length, using RFC 7230 Section 3.3. This
 // function is not a method, because ultimately it should be shared by
 // ReadResponse and ReadRequest.
-func fixLength(isResponse bool, status int, requestMethod string, header Header, chunked bool) (int64, error) {
+func fixLength(isResponse bool, status int, requestMethod string, header Header, chunked bool) (n int64, err error) {
 	isRequest := !isResponse
 	contentLens := header["Content-Length"]
 
@@ -694,6 +681,14 @@ func fixLength(isResponse bool, status int, requestMethod string, header Header,
 		contentLens = header["Content-Length"]
 	}
 
+	// Reject requests with invalid Content-Length headers.
+	if len(contentLens) > 0 {
+		n, err = parseContentLength(contentLens)
+		if err != nil {
+			return -1, err
+		}
+	}
+
 	// Logic based on response type or status
 	if isResponse && noResponseBodyExpected(requestMethod) {
 		return 0, nil
@@ -706,17 +701,26 @@ func fixLength(isResponse bool, status int, requestMethod string, header Header,
 		return 0, nil
 	}
 
+	// According to RFC 9112, "If a message is received with both a
+	// Transfer-Encoding and a Content-Length header field, the Transfer-Encoding
+	// overrides the Content-Length. Such a message might indicate an attempt to
+	// perform request smuggling (Section 11.2) or response splitting (Section 11.1)
+	// and ought to be handled as an error. An intermediary that chooses to forward
+	// the message MUST first remove the received Content-Length field and process
+	// the Transfer-Encoding (as described below) prior to forwarding the message downstream."
+	//
+	// Chunked-encoding requests with either valid Content-Length
+	// headers or no Content-Length headers are accepted after removing
+	// the Content-Length field from header.
+	//
 	// Logic based on Transfer-Encoding
 	if chunked {
+		header.Del("Content-Length")
 		return -1, nil
 	}
 
+	// Logic based on Content-Length
 	if len(contentLens) > 0 {
-		// Logic based on Content-Length
-		n, err := parseContentLength(contentLens)
-		if err != nil {
-			return -1, err
-		}
 		return n, nil
 	}