J2SE Version (please include all output from java -version flag): java version "1.6.0_23" Java(TM) SE Runtime Environment (build 1.6.0_23-b05) Java HotSpot(TM) Client VM (build 19.0-b09, mixed mode, sharing) Does this problem occur on J2SE 5.0.x or 6ux ? Yes / No (pick one) Java 6ux Operating System Configuration Information (be specific): Windows XP with both IE 8 and Firefox 3.6 Bug Description: Java Plugin: Loading applet-JARs with SSL and client authentication causes ConcurrentModificationException . It is a regression problem and occurs only with the next-generation java-plugin Steps to Reproduce (be specific): Deploy an applet on a site where access is restricted to ssl client authentication. Launch the applet and watch the java console. (check Comments field for detail) EXPECTED VERSUS ACTUAL BEHAVIOR : EXPECTED - Expected Console Output No error is logged ACTUAL - Actual Console Output The console output starts with Exception in thread "HandshakeCompletedNotify-Thread" java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextEntry(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at com.sun.net.ssl.internal.ssl.SSLSocketImpl$NotifyHandshakeThread.run(Unknown Source) This error appears after you choose your ssl client certificate in the java plugin client authentication dialog. REPRODUCIBILITY : This bug can be reproduced always.