From 345408d2044fcd4338892cc6a578d01ae1fafe65 Mon Sep 17 00:00:00 2001
From: "William T. Nelson" <35801+wtn@users.noreply.github.com>
Date: Mon, 2 Nov 2026 12:38:22 -0600
Subject: [PATCH] Truncate close frame reasons to fit in a control frame.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
---
 lib/protocol/websocket/close_frame.rb  |  6 ++++++
 test/protocol/websocket/close_frame.rb | 19 +++++++++++++++++++
 test/protocol/websocket/connection.rb  | 11 +++++++++++
 3 files changed, 36 insertions(+)

diff --git a/lib/protocol/websocket/close_frame.rb b/lib/protocol/websocket/close_frame.rb
index f718f22..ad53caa 100644
--- a/lib/protocol/websocket/close_frame.rb
+++ b/lib/protocol/websocket/close_frame.rb
@@ -12,6 +12,7 @@ module WebSocket
 		class CloseFrame < Frame
 			OPCODE = 0x8
 			FORMAT = "na*"
+			MAXIMUM_REASON_LENGTH = 123
 			
 			# Unpack the frame data into a close code and reason.
 			# @returns [Tuple(Integer, String)] The close code and reason.
@@ -45,6 +46,7 @@ def unpack
 			
 			# Pack a close code and reason into the frame data.
 			# If code is missing, reason is ignored.
+			# The reason is truncated to 123 bytes.
 			# @parameter code [Integer | Nil] The close code.
 			# @parameter reason [String | Nil] The close reason.
 			def pack(code = nil, reason = nil)
@@ -53,6 +55,10 @@ def pack(code = nil, reason = nil)
 						reason = reason.encode(Encoding::UTF_8)
 					end
 					
+					if reason and reason.bytesize > MAXIMUM_REASON_LENGTH
+						reason = reason.byteslice(0, MAXIMUM_REASON_LENGTH).scrub("")
+					end
+					
 					super([code, reason].pack(FORMAT))
 				else
 					super()
diff --git a/test/protocol/websocket/close_frame.rb b/test/protocol/websocket/close_frame.rb
index c70b46e..4402cd4 100644
--- a/test/protocol/websocket/close_frame.rb
+++ b/test/protocol/websocket/close_frame.rb
@@ -30,6 +30,25 @@
 			
 			expect(frame.payload).to be == "\x03\xE8Hello World".b
 		end
+		
+		it "truncates the reason to fit in a control frame" do
+			frame.pack(1000, "x" * 200)
+			
+			expect(frame.payload.bytesize).to be == 125
+			
+			code, reason = frame.unpack
+			expect(code).to be == 1000
+			expect(reason).to be == "x" * 123
+		end
+		
+		it "doesn't split a multibyte character when truncating the reason" do
+			frame.pack(1000, ("x" * 122) + "é")
+			
+			expect(frame.payload.bytesize).to be == 124
+			
+			code, reason = frame.unpack
+			expect(reason).to be == "x" * 122
+		end
 	end
 	
 	with "#unpack" do
diff --git a/test/protocol/websocket/connection.rb b/test/protocol/websocket/connection.rb
index 158bddf..7d3df0e 100644
--- a/test/protocol/websocket/connection.rb
+++ b/test/protocol/websocket/connection.rb
@@ -69,6 +69,17 @@
 			expect(connection).to be(:closed?)
 		end
 		
+		it "truncates a long error message to fit in the close frame" do
+			connection.close_write(RuntimeError.new("x" * 200))
+			
+			frame = client.read_frame
+			expect(frame).to be_a(Protocol::WebSocket::CloseFrame)
+			
+			code, reason = frame.unpack
+			expect(code).to be == Protocol::WebSocket::Error::INTERNAL_ERROR
+			expect(reason).to be == "x" * 123
+		end
+		
 		it "is not closed when close_write succeeds" do
 			connection.close_write