diff --git a/pkg/proxy/fast/proxy.go b/pkg/proxy/fast/proxy.go
index a400ce646..717b1ff06 100644
--- a/pkg/proxy/fast/proxy.go
+++ b/pkg/proxy/fast/proxy.go
@@ -171,7 +171,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	if reqUpType != "" {
 		outReq.Header.Set("Connection", "Upgrade")
 		outReq.Header.Set("Upgrade", reqUpType)
-		if reqUpType == "websocket" {
+		if strings.EqualFold(reqUpType, "websocket") {
 			cleanWebSocketHeaders(&outReq.Header)
 		}
 	}
@@ -353,6 +353,7 @@ type fasthttpHeader interface {
 	SetBytesV(key string, value []byte)
 	DelBytes(key []byte)
 	Del(key string)
+	ConnectionUpgrade() bool
 }
 
 // removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
diff --git a/pkg/proxy/fast/proxy_websocket_test.go b/pkg/proxy/fast/proxy_websocket_test.go
index b057f8b58..ef22895cc 100644
--- a/pkg/proxy/fast/proxy_websocket_test.go
+++ b/pkg/proxy/fast/proxy_websocket_test.go
@@ -2,7 +2,9 @@ package fast
 
 import (
 	"bufio"
+	"crypto/sha1"
 	"crypto/tls"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"net"
@@ -19,6 +21,34 @@ import (
 	"golang.org/x/net/websocket"
 )
 
+func TestWebSocketUpgradeCase(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		challengeKey := r.Header.Get("Sec-Websocket-Key")
+
+		hijacker, ok := w.(http.Hijacker)
+		require.True(t, ok)
+
+		c, _, err := hijacker.Hijack()
+		require.NoError(t, err)
+
+		// Force answer with "Connection: upgrade" in lowercase.
+		_, err = c.Write([]byte("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: upgrade\r\nSec-WebSocket-Accept: " + computeAcceptKey(challengeKey) + "\r\n\n"))
+		require.NoError(t, err)
+	}))
+	defer srv.Close()
+
+	proxy := createProxyWithForwarder(t, srv.URL, createConnectionPool(srv.URL, nil))
+
+	proxyAddr := proxy.Listener.Addr().String()
+	_, conn, err := newWebsocketRequest(
+		withServer(proxyAddr),
+		withPath("/ws"),
+	).open()
+	require.NoError(t, err)
+
+	conn.Close()
+}
+
 func TestWebSocketTCPClose(t *testing.T) {
 	errChan := make(chan error, 1)
 	upgrader := gorillawebsocket.Upgrader{}
@@ -691,3 +721,10 @@ func createProxyWithForwarder(t *testing.T, uri string, pool *connPool) *httptes
 
 	return srv
 }
+
+func computeAcceptKey(challengeKey string) string {
+	h := sha1.New() // #nosec G401 -- (CWE-326) https://datatracker.ietf.org/doc/html/rfc6455#page-54
+	h.Write([]byte(challengeKey))
+	h.Write([]byte("258EAFA5-E914-47DA-95CA-C5AB0DC85B11"))
+	return base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
diff --git a/pkg/proxy/fast/upgrade.go b/pkg/proxy/fast/upgrade.go
index 7bec09e49..2570d4c72 100644
--- a/pkg/proxy/fast/upgrade.go
+++ b/pkg/proxy/fast/upgrade.go
@@ -1,7 +1,6 @@
 package fast
 
 import (
-	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -100,7 +99,7 @@ func upgradeType(h http.Header) string {
 }
 
 func upgradeTypeFastHTTP(h fasthttpHeader) string {
-	if !bytes.Contains(h.Peek("Connection"), []byte("Upgrade")) {
+	if !h.ConnectionUpgrade() {
 		return ""
 	}