diff --git a/common/buf/writer.go b/common/buf/writer.go
index 66d24b0e2..a6d4bca00 100644
--- a/common/buf/writer.go
+++ b/common/buf/writer.go
@@ -3,6 +3,7 @@ package buf
 import (
 	"io"
 	"net"
+	"sync"
 
 	"v2ray.com/core/common"
 	"v2ray.com/core/common/errors"
@@ -56,6 +57,7 @@ func (w *BufferToBytesWriter) ReadFrom(reader io.Reader) (int64, error) {
 
 // BufferedWriter is a Writer with internal buffer.
 type BufferedWriter struct {
+	sync.Mutex
 	writer   Writer
 	buffer   *Buffer
 	buffered bool
@@ -77,6 +79,9 @@ func (w *BufferedWriter) WriteByte(c byte) error {
 
 // Write implements io.Writer.
 func (w *BufferedWriter) Write(b []byte) (int, error) {
+	w.Lock()
+	defer w.Unlock()
+
 	if !w.buffered {
 		if writer, ok := w.writer.(io.Writer); ok {
 			return writer.Write(b)
@@ -95,7 +100,7 @@ func (w *BufferedWriter) Write(b []byte) (int, error) {
 			return totalBytes, err
 		}
 		if !w.buffered || w.buffer.IsFull() {
-			if err := w.Flush(); err != nil {
+			if err := w.flushInternal(); err != nil {
 				return totalBytes, err
 			}
 		}
@@ -107,6 +112,9 @@ func (w *BufferedWriter) Write(b []byte) (int, error) {
 
 // WriteMultiBuffer implements Writer. It takes ownership of the given MultiBuffer.
 func (w *BufferedWriter) WriteMultiBuffer(b MultiBuffer) error {
+	w.Lock()
+	defer w.Unlock()
+
 	if !w.buffered {
 		return w.writer.WriteMultiBuffer(b)
 	}
@@ -121,7 +129,7 @@ func (w *BufferedWriter) WriteMultiBuffer(b MultiBuffer) error {
 			return err
 		}
 		if w.buffer.IsFull() {
-			if err := w.Flush(); err != nil {
+			if err := w.flushInternal(); err != nil {
 				return err
 			}
 		}
@@ -132,6 +140,13 @@ func (w *BufferedWriter) WriteMultiBuffer(b MultiBuffer) error {
 
 // Flush flushes buffered content into underlying writer.
 func (w *BufferedWriter) Flush() error {
+	w.Lock()
+	defer w.Unlock()
+
+	return w.flushInternal()
+}
+
+func (w *BufferedWriter) flushInternal() error {
 	if w.buffer.IsEmpty() {
 		return nil
 	}
@@ -150,9 +165,12 @@ func (w *BufferedWriter) Flush() error {
 
 // SetBuffered sets whether the internal buffer is used. If set to false, Flush() will be called to clear the buffer.
 func (w *BufferedWriter) SetBuffered(f bool) error {
+	w.Lock()
+	defer w.Unlock()
+
 	w.buffered = f
 	if !f {
-		return w.Flush()
+		return w.flushInternal()
 	}
 	return nil
 }