Your problem isn't on the shown code. Your problem is on you class Store.
EXPLANATION: What is happening is that while you iterate over Store.getAll() sockets, another socket is being inserted or removed from the same structure retrieved by Store.getAll() (much likely subclass of java.util.Collection - e.g.: LinkedList, ArrayList,HashMap, etc..).
REASON: You cannot iterate over Collection while the same collection is being modified. See ConcurrentModificationException javadocs
SOLUTION: Anyway, to solve your problem on the snippet shown, I suggest that you clone the structure returned by Store.getAll(). But be aware that this structure will be obsolete on its content, because there are sockets being inserted or removed in your Store class.
TIP: (this won't solve your problem but will help you to program fashionably) To iterate over elements from some structure use this in Java:
for(Socket s : Store.getAll()) {
try {
if (s.getInputStream().available() != 0) {
byte[] buffer = new byte[s.getInputStream().available()];
s.getInputStream().read(buffer);
new Processing(s, buffer);
}
} catch (IOException e) {
e.printStackTrace();
}
}