diff --git a/proxy/http/server.go b/proxy/http/server.go
index b89c67c0ed43..349ba7dde647 100644
--- a/proxy/http/server.go
+++ b/proxy/http/server.go
@@ -312,8 +312,9 @@ func (s *Server) handlePlainHTTP(ctx context.Context, request *http.Request, wri
 // Sometimes, server might send 1xx response to client
 // it should not be processed by http proxy handler, just forward it to client
 func readResponseAndHandle100Continue(r *bufio.Reader, req *http.Request, writer io.Writer) (*http.Response, error) {
-	// have a little look of response
-	peekBytes, err := r.Peek(56)
+	// Wait for the HTTP/1.x status prefix before inspecting the buffered response.
+	r.Peek(12)
+	peekBytes, err := r.Peek(min(56, r.Buffered()))
 	if err == nil || err == bufio.ErrBufferFull {
 		str := string(peekBytes)
 		ResponseLine := strings.Split(str, "\r\n")[0]
diff --git a/proxy/http/server_test.go b/proxy/http/server_test.go
index 2578d5315e97..5b3563315de8 100644
--- a/proxy/http/server_test.go
+++ b/proxy/http/server_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"strings"
 	"testing"
+	"testing/iotest"
 )
 
 // A malformed upstream response containing a bare '\n' before the real
@@ -29,7 +30,7 @@ func TestReadResponseAndHandle100ContinueDoesNotPanicOnEarlyNewline(t *testing.T
 func TestReadResponseAndHandle100ContinueForwardsAndParsesFinalResponse(t *testing.T) {
 	payload := "HTTP/1.1 100 Continue\r\n\r\n" +
 		"HTTP/1.1 200 OK\r\nContent-Length: 5\r\n\r\nhello"
-	r := bufio.NewReader(bytes.NewReader([]byte(payload)))
+	r := bufio.NewReader(iotest.OneByteReader(bytes.NewReader([]byte(payload))))
 	req, err := http.NewRequest("GET", "http://example.com/", nil)
 	if err != nil {
 		t.Fatal(err)