diff --git a/UPSTREAM b/UPSTREAM
index 3e9ef2da..88edebf1 100644
--- a/UPSTREAM
+++ b/UPSTREAM
@@ -1 +1 @@
-go1.21.8
+go1.21.9
diff --git a/h2_bundle.go b/h2_bundle.go
index 2946bc92..0d7f8ee7 100644
--- a/h2_bundle.go
+++ b/h2_bundle.go
@@ -2966,6 +2966,7 @@ func (fr *http2Framer) readMetaFrame(hf *http2HeadersFrame) (*http2MetaHeadersFr
 		if size > remainSize {
 			hdec.SetEmitEnabled(false)
 			mh.Truncated = true
+			remainSize = 0
 			return
 		}
 		remainSize -= size
@@ -2978,6 +2979,36 @@ func (fr *http2Framer) readMetaFrame(hf *http2HeadersFrame) (*http2MetaHeadersFr
 	var hc http2headersOrContinuation = hf
 	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 http2VerboseLogs {
+				log.Printf("http2: header list too large")
+			}
+			// It would be nice to send a RST_STREAM before sending the GOAWAY,
+			// but the struture of the server's frame writer makes this difficult.
+			return nil, http2ConnectionError(http2ErrCodeProtocol)
+		}
+
+		// 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 http2VerboseLogs {
+				log.Printf("http2: invalid header: %v", invalid)
+			}
+			// It would be nice to send a RST_STREAM before sending the GOAWAY,
+			// but the struture of the server's frame writer makes this difficult.
+			return nil, http2ConnectionError(http2ErrCodeProtocol)
+		}
+
 		if _, err := hdec.Write(frag); err != nil {
 			return nil, http2ConnectionError(http2ErrCodeCompression)
 		}
@@ -9748,7 +9779,7 @@ func (rl *http2clientConnReadLoop) processData(f *http2DataFrame) error {
 		})
 		return nil
 	}
-	if !cs.firstByte {
+	if !cs.pastHeaders {
 		cc.logf("protocol error: received DATA before a HEADERS frame")
 		rl.endStreamError(cs, http2StreamError{
 			StreamID: f.StreamID,