double leading dot on first line in DotTerminatedMessageWriter

src/main/java/org/apache/commons/net/io/DotTerminatedMessageWriter.java

@@ -45,7 +45,9 @@ public final class DotTerminatedMessageWriter extends Writer {
     public DotTerminatedMessageWriter(final Writer output) {
         super(output);
         this.output = output;
-        this.state = NOTHING_SPECIAL_STATE;
+        // The message starts at the beginning of a line, so a leading dot on the
+        // first line must be doubled just like on any other line.
+        this.state = LAST_WAS_NL_STATE;
     }
 
     /**

src/test/java/org/apache/commons/net/io/DotTerminatedMessageWriterTest.java

@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.net.io;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.IOException;
+import java.io.StringWriter;
+
+import org.junit.jupiter.api.Test;
+
+class DotTerminatedMessageWriterTest {
+
+    private String write(final String message) throws IOException {
+        final StringWriter sw = new StringWriter();
+        try (DotTerminatedMessageWriter writer = new DotTerminatedMessageWriter(sw)) {
+            writer.write(message);
+        }
+        return sw.toString();
+    }
+
+    @Test
+    void testDoubleLeadingDotOnFirstLine() throws IOException {
+        // A leading dot on the first line must be doubled, otherwise it is
+        // indistinguishable from the message terminator.
+        assertEquals("..\r\n.\r\n", write("."));
+        assertEquals("..hidden\r\n.\r\n", write(".hidden"));
+    }
+
+    @Test
+    void testFirstLineDotCannotTerminateEarly() throws IOException {
+        // "." CR LF as the first line would otherwise end the message and let the
+        // following text be read as protocol commands.
+        assertEquals("..\r\nINJECT\r\n.\r\n", write(".\r\nINJECT"));
+    }
+
+    @Test
+    void testDoubleLeadingDotOnLaterLine() throws IOException {
+        assertEquals("a\r\n..b\r\n.\r\n", write("a\r\n.b"));
+    }
+}