1 files changed, 31 insertions, 6 deletions
diff --git a/internal/net/conn.go b/internal/net/conn.go
index e46031b..2fff0bc 100644
--- a/internal/net/conn.go
+++ b/internal/net/conn.go
@@ -24,21 +24,46 @@ type tcpConn struct {
 func newTCPConn(conn net.Conn) *tcpConn {
 	return &tcpConn{
 		conn:   conn,
-		reader: bufio.NewReader(conn),
+		reader: bufio.NewReaderSize(conn, maxLineLength*4),
 	}
 }
 
+// maxLineLength bounds how much of an incoming line is accepted. Longer
+// lines are drained from the socket and truncated so a client cannot grow
+// the read buffer without limit.
+const maxLineLength = 1024
+
 func (t *tcpConn) ReadMessage() (string, error) {
-	line, err := t.reader.ReadString('\n')
-	if err != nil {
+	line, err := t.reader.ReadSlice('\n')
+	if err == bufio.ErrBufferFull {
+		// Copy the bounded prefix out of the reusable read buffer BEFORE
+		// draining the rest of the oversized line; further reads overwrite the
+		// buffer the slice points into.
+		prefixLen := len(line)
+		if prefixLen > maxLineLength {
+			prefixLen = maxLineLength
+		}
+		prefix := make([]byte, prefixLen)
+		copy(prefix, line[:prefixLen])
+		for {
+			_, err = t.reader.ReadSlice('\n')
+			if err == nil {
+				break
+			}
+			if err != bufio.ErrBufferFull {
+				return "", err
+			}
+		}
+		line = prefix
+	} else if err != nil {
 		return "", err
 	}
-	line = strings.TrimRight(line, "\r\n")
-	line = stripIAC(line)
+	text := strings.TrimRight(string(line), "\r\n")
+	text = stripIAC(text)
 	if t.echoOff {
 		t.conn.Write([]byte("\r\n"))
 	}
-	return line, nil
+	return text, nil
 }
 
 func (t *tcpConn) Write(b []byte) (int, error) {