Exception in Internet-headers when parsing malformed MIME headers #827

api/src/main/java/jakarta/mail/internet/InternetHeaders.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2025 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2026 Oracle and/or its affiliates. All rights reserved.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0, which is available at
@@ -660,8 +660,10 @@ public void addHeaderLine(String line) {
         try {
             char c = line.charAt(0);
             if (c == ' ' || c == '\t') {
-                InternetHeader h = headers.get(headers.size() - 1);
-                h.line += "\r\n" + line;
+                if (!headers.isEmpty()) {
+                    InternetHeader h = headers.get(headers.size() - 1);
+                    h.line += "\r\n" + line;
+                }
             } else
                 headers.add(new InternetHeader(line));
         } catch (StringIndexOutOfBoundsException e) {

api/src/test/java/jakarta/mail/internet/MimeMessageTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, 2026 Oracle and/or its affiliates. All rights reserved.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0, which is available at
@@ -19,6 +19,8 @@
 
 import static org.junit.Assert.assertEquals;
 
+import java.io.ByteArrayInputStream;
+import java.util.Base64;
 import java.util.Properties;
 import jakarta.mail.Session;
 import org.junit.Test;
@@ -83,4 +85,13 @@ public void testSystemPropertiesAndSessionWithNoProperties() throws Exception {
             System.clearProperty("mail.mime.allowutf8");
         }
     }
+
+    @Test
+    public void testEmptyHeaders() throws Exception {
+        String base64Input = "ICAgIAogICAKCg==";
+        byte[] input = Base64.getDecoder().decode(base64Input);
+        Session session = Session.getDefaultInstance(new Properties());
+        ByteArrayInputStream is = new ByteArrayInputStream(input);
+        new MimeMessage(session, is);
+    }
 }

doc/release/CHANGES.txt

@@ -21,6 +21,7 @@ longer available.
 
       CHANGES IN THE 2.2.0 RELEASE
       ----------------------------
+E  827 Exception in Internet-headers when parsing malformed MIME headers
 E  714 Support minimum JDK of 11+ and minimum of Android API 33+
 E  720 finalize() in java.lang.Object has been deprecated
 E  744 Remove SecurityManager reference from API