From ac87c4e93557619f326275fbc7ef7431adecca4f Mon Sep 17 00:00:00 2001
From: AshSgDe29071999 <AshSgDe29071999@users.noreply.github.com>
Date: Mon, 31 Aug 2026 14:57:09 +0530
Subject: [PATCH] Bound CloseError.Error text to the control-frame limit

Error() appended Text without a length cap. A CloseError with a huge
Text (corrupt state or a caller-built value) could OOM while formatting
the error. Truncate to maxControlFramePayloadSize, the protocol maximum
for close-frame payloads.

See #1002
---
 conn.go      |  6 +++++-
 conn_test.go | 13 +++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/conn.go b/conn.go
index 9562ffd4..38b9b5a6 100644
--- a/conn.go
+++ b/conn.go
@@ -140,7 +140,11 @@ func (e *CloseError) Error() string {
 	}
 	if e.Text != "" {
 		s = append(s, ": "...)
-		s = append(s, e.Text...)
+		text := e.Text
+		if len(text) > maxControlFramePayloadSize {
+			text = text[:maxControlFramePayloadSize]
+		}
+		s = append(s, text...)
 	}
 	return string(s)
 }
diff --git a/conn_test.go b/conn_test.go
index 28f5c4a3..51cf15e0 100644
--- a/conn_test.go
+++ b/conn_test.go
@@ -12,6 +12,7 @@ import (
 	"io"
 	"net"
 	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"testing/iotest"
@@ -671,6 +672,18 @@ func TestCloseError(t *testing.T) {
 	}
 }
 
+func TestCloseErrorTextIsBounded(t *testing.T) {
+	huge := strings.Repeat("x", 1<<20)
+	err := &CloseError{Code: CloseNormalClosure, Text: huge}
+	msg := err.Error()
+	if len(msg) > 64+maxControlFramePayloadSize {
+		t.Fatalf("Error() length %d exceeds bound", len(msg))
+	}
+	if !strings.Contains(msg, "websocket: close 1000") {
+		t.Fatalf("Error() = %q", msg)
+	}
+}
+
 var unexpectedCloseErrorTests = []struct {
 	err   error
 	codes []int