diff --git a/internal/api/websocket.go b/internal/api/websocket.go
index c5d2e1b..9f201ba 100644
--- a/internal/api/websocket.go
+++ b/internal/api/websocket.go
@@ -10,6 +10,7 @@ import (
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
 	"io"
 	"net/http"
+	"sync"
 )
 
 const CodeOceanToRawReaderBufferSize = 1024
@@ -161,11 +162,12 @@ func (rc *rawToCodeOceanWriter) Write(p []byte) (int, error) {
 
 // webSocketProxy is an encapsulation of logic for forwarding between Runners and CodeOcean.
 type webSocketProxy struct {
-	userExit   chan bool
-	connection webSocketConnection
-	Stdin      WebSocketReader
-	Stdout     io.Writer
-	Stderr     io.Writer
+	userExit     chan bool
+	connection   webSocketConnection
+	connectionMu sync.Mutex
+	Stdin        WebSocketReader
+	Stdout       io.Writer
+	Stderr       io.Writer
 }
 
 // upgradeConnection upgrades a connection to a websocket and returns a webSocketProxy for this connection.
@@ -261,7 +263,7 @@ func (wp *webSocketProxy) sendToClient(message dto.WebSocketMessage) error {
 		wp.closeWithError("Error creating message")
 		return fmt.Errorf("error marshaling WebSocket message: %w", err)
 	}
-	err = wp.connection.WriteMessage(websocket.TextMessage, encodedMessage)
+	err = wp.writeMessage(websocket.TextMessage, encodedMessage)
 	if err != nil {
 		errorMessage := "Error writing the exit message"
 		log.WithError(err).Warn(errorMessage)
@@ -280,13 +282,19 @@ func (wp *webSocketProxy) closeNormal() {
 }
 
 func (wp *webSocketProxy) close(message []byte) {
-	err := wp.connection.WriteMessage(websocket.CloseMessage, message)
+	err := wp.writeMessage(websocket.CloseMessage, message)
 	_ = wp.connection.Close()
 	if err != nil {
 		log.WithError(err).Warn("Error during websocket close")
 	}
 }
 
+func (wp *webSocketProxy) writeMessage(messageType int, data []byte) error {
+	wp.connectionMu.Lock()
+	defer wp.connectionMu.Unlock()
+	return wp.connection.WriteMessage(messageType, data) //nolint:wrapcheck // Wrap the original WriteMessage in a mutex.
+}
+
 // connectToRunner is the endpoint for websocket connections.
 func (r *RunnerController) connectToRunner(writer http.ResponseWriter, request *http.Request) {
 	targetRunner, _ := runner.FromContext(request.Context())