http2: close connections when receiving too many headers (#1156)

Maintaining HPACK state requires that we parse and process all HEADERS and CONTINUATION frames on a connection. When a request's headers exceed MaxHeaderBytes, we don't allocate memory to store the excess headers but we do parse them. This permits an attacker to cause an HTTP/2 endpoint to read arbitrary amounts of header data, all associated with a request which is going to be rejected. These headers can include Huffman-encoded data which is significantly more expensive for the receiver to decode than for an attacker to send.

Set a limit on the amount of excess header frames we will process before closing a connection.

This is CVE-2023-45288 and Go issue https://go.dev/issue/65051.

bfe_http2/frame.go

@@ -1504,6 +1504,7 @@ func (fr *Framer) readMetaFrame(f *HeadersFrame) (*MetaHeadersFrame, error) {
 			hdec.SetEmitEnabled(false)
 			mh.Truncated = true
 			state.H2ErrMaxHeaderListSize.Inc(1)
+			remainSize = 0
 			return maxHeaderListSizeError{
 				streamID:          f.FrameHeader.StreamID,
 				curHeaderListSize: fr.maxHeaderListSize() - remainSize + size,
@@ -1525,6 +1526,35 @@ func (fr *Framer) readMetaFrame(f *HeadersFrame) (*MetaHeadersFrame, error) {
 	var err error
 	for {
 		frag := hc.HeaderBlockFragment()
+
+		// Avoid parsing large amounts of headers that we will then discard.
+		// If the sender exceeds the max header list size by too much,
+		// skip parsing the fragment and close the connection.
+		//
+		// "Too much" is either any CONTINUATION frame after we've already
+		// exceeded the max header list size (in which case remainSize is 0),
+		// or a frame whose encoded size is more than twice the remaining
+		// header list bytes we're willing to accept.
+		if int64(len(frag)) > int64(2*remainSize) {
+			if VerboseLogs {
+				log.Printf("http2: header list too large")
+			}
+			// It would be nice to send a RST_STREAM before sending the GOAWAY,
+			// but the structure of the server's frame writer makes this difficult.
+			return nil, ConnectionError{ErrCodeProtocol, "http2: header list too large"}
+		}
+
+		// Also close the connection after any CONTINUATION frame following an
+		// invalid header, since we stop tracking the size of the headers after
+		// an invalid one.
+		if invalid != nil {
+			if VerboseLogs {
+				log.Printf("http2: invalid header: %v", invalid)
+			}
+			// It would be nice to send a RST_STREAM before sending the GOAWAY,
+			// but the structure of the server's frame writer makes this difficult.
+			return nil, ConnectionError{ErrCodeProtocol, fmt.Sprintf("http2: invalid header: %v", invalid)}
+		}
 		blockSize += len(frag)
 		if _, err = hdec.Write(frag); err != nil {
 			// do not return ConnectionError err type,

bfe_http2/frame_test.go

@@ -931,7 +931,7 @@ func TestMetaFrameHeader(t *testing.T) {
 			maxHeaderListSize: (1 << 10) / 2,
 			want: maxHeaderListSizeError{
 				streamID:          1,
-				curHeaderListSize: 536,
+				curHeaderListSize: 550,
 				maxHeaderListSize: 512,
 			},
 		},
@@ -1033,7 +1033,7 @@ func TestMetaFrameHeader(t *testing.T) {
 				}
 				return fmt.Sprintf("value %#v", v)
 			}
-			t.Errorf("%s:\n got: %v\nwant: %s", name, str(got), str(tt.want))
+			t.Errorf(" %s:\n got: %v\nwant: %s", name, str(got), str(tt.want))
 		}
 		if tt.wantErrReason != "" && tt.wantErrReason != fmt.Sprint(f.errDetail) {
 			t.Errorf("%s: got error reason %q; want %q", name, f.errDetail, tt.wantErrReason)