net/http: disallow empty Content-Length header

The Content-Length must be a valid numeric value, empty values should
not be accepted.

See: https://www.rfc-editor.org/rfc/rfc9110.html#name-content-length

Fixes #61679

Author: mauri870

src/internal/godebugs/table.go

@@ -32,6 +32,7 @@ var All = []Info{
 	{Name: "http2client", Package: "net/http"},
 	{Name: "http2debug", Package: "net/http", Opaque: true},
 	{Name: "http2server", Package: "net/http"},
+	{Name: "httplaxcontentlength", Package: "net/http"},
 	{Name: "installgoroot", Package: "go/build"},
 	{Name: "jstmpllitinterp", Package: "html/template"},
 	//{Name: "multipartfiles", Package: "mime/multipart"},

src/net/http/response_test.go

@@ -883,6 +883,7 @@ func TestReadResponseErrors(t *testing.T) {
 	}
 
 	errMultiCL := "message cannot contain multiple Content-Length headers"
+	errEmptyCL := "invalid empty Content-Length"
 
 	tests := []testCase{
 		{"", "", io.ErrUnexpectedEOF},
@@ -918,7 +919,7 @@ func TestReadResponseErrors(t *testing.T) {
 		contentLength("200 OK", "Content-Length: 7\r\nContent-Length: 7\r\n\r\nGophers\r\n", nil),
 		contentLength("201 OK", "Content-Length: 0\r\nContent-Length: 7\r\n\r\nGophers\r\n", errMultiCL),
 		contentLength("300 OK", "Content-Length: 0\r\nContent-Length: 0 \r\n\r\nGophers\r\n", nil),
-		contentLength("200 OK", "Content-Length:\r\nContent-Length:\r\n\r\nGophers\r\n", nil),
+		contentLength("200 OK", "Content-Length:\r\nContent-Length:\r\n\r\nGophers\r\n", errEmptyCL),
 		contentLength("206 OK", "Content-Length:\r\nContent-Length: 0 \r\nConnection: close\r\n\r\nGophers\r\n", errMultiCL),
 
 		// multiple content-length headers for 204 and 304 should still be checked

src/net/http/transfer.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"internal/godebug"
 	"io"
 	"net/http/httptrace"
 	"net/http/internal"
@@ -527,7 +528,7 @@ func readTransfer(msg any, r *bufio.Reader) (err error) {
 		return err
 	}
 	if isResponse && t.RequestMethod == "HEAD" {
-		if n, err := parseContentLength(t.Header.get("Content-Length")); err != nil {
+		if n, err := parseContentLength(t.Header["Content-Length"]); err != nil {
 			return err
 		} else {
 			t.ContentLength = n
@@ -707,18 +708,15 @@ func fixLength(isResponse bool, status int, requestMethod string, header Header,
 		return -1, nil
 	}
 
-	// Logic based on Content-Length
-	var cl string
-	if len(contentLens) == 1 {
-		cl = textproto.TrimString(contentLens[0])
-	}
-	if cl != "" {
-		n, err := parseContentLength(cl)
+	if len(contentLens) > 0 {
+		// Logic based on Content-Length
+		n, err := parseContentLength(contentLens)
 		if err != nil {
 			return -1, err
 		}
 		return n, nil
 	}
+
 	header.Del("Content-Length")
 
 	if isRequest {
@@ -1038,19 +1036,29 @@ func (bl bodyLocked) Read(p []byte) (n int, err error) {
 	return bl.b.readLocked(p)
 }
 
+var laxContentLength = godebug.New("httplaxcontentlength")
+
 // parseContentLength trims whitespace from s and returns -1 if no value
-// is set, or the value if it's >= 0.
-func parseContentLength(cl string) (int64, error) {
-	cl = textproto.TrimString(cl)
-	if cl == "" {
+// is set otherwise the value if it's >= 0.
+func parseContentLength(clHeaders []string) (int64, error) {
+	if len(clHeaders) == 0 {
 		return -1, nil
 	}
+	cl := textproto.TrimString(clHeaders[0])
+
+	// The Content-Length must be a valid numeric value.
+	// See: https://datatracker.ietf.org/doc/html/rfc2616/#section-14.13
+	if cl == "" {
+		if laxContentLength.Value() == "1" {
+			return -1, nil
+		}
+		return 0, badStringError("invalid empty Content-Length", cl)
+	}
 	n, err := strconv.ParseUint(cl, 10, 63)
 	if err != nil {
 		return 0, badStringError("bad Content-Length", cl)
 	}
 	return int64(n), nil
-
 }
 
 // finishAsyncByteRead finishes reading the 1-byte sniff

src/net/http/transfer_test.go

@@ -332,6 +332,10 @@ func TestParseContentLength(t *testing.T) {
 		cl      string
 		wantErr error
 	}{
+		{
+			cl:      "",
+			wantErr: badStringError("invalid empty Content-Length", ""),
+		},
 		{
 			cl:      "3",
 			wantErr: nil,
@@ -356,7 +360,7 @@ func TestParseContentLength(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		if _, gotErr := parseContentLength(tt.cl); !reflect.DeepEqual(gotErr, tt.wantErr) {
+		if _, gotErr := parseContentLength([]string{tt.cl}); !reflect.DeepEqual(gotErr, tt.wantErr) {
 			t.Errorf("%q:\n\tgot=%v\n\twant=%v", tt.cl, gotErr, tt.wantErr)
 		}
 	}