DOS against Java JNDI/DNS

[Kurt Huwig <k.huwig@xxxxxxxxx>]

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_______________________________________________________________________

~                           iKu Advisory
_______________________________________________________________________

Product               : Java Runtime Environment
Date                  : November 8th 2004
Affected versions     : 1.4.2, 1.5.0, probably more
Vulnerability Type    : remote denial of service
Severity (1-10)       : 3
Remote                : yes
_______________________________________________________________________

0. contents

~  1. problem description
~  2. symptoms
~  3. bug description/possible fix
~  4. sample code
~  5. workaround


1. problem description

Java uses an 'InitialDirContext' to perform DNS lookups. A wrap-around
of an internal variable renders the context unusable after 32768
requests for the next 32768 requests. A remote attacker might trick an
application into performing a lot of DNS requests which renders the
application unusable. It might also be the cause of some problems with
long running server processes.


2. symptoms

When the 32768 requests are done, the following requests throw this
exception:

javax.naming.CommunicationException: DNS error: ID doesn't match;
remaining name 'sun.com'
        at com.sun.jndi.dns.DnsClient.checkHeader(DnsClient.java:426)
        at com.sun.jndi.dns.DnsClient.query(DnsClient.java:162)
        at com.sun.jndi.dns.Resolver.query(Resolver.java:63)
        at com.sun.jndi.dns.DnsContext.c_getAttributes(DnsContext.java:410)
        at
com.sun.jndi.toolkit.ctx.ComponentDirContext.p_getAttributes(ComponentDirContext.java:213)
        at
com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.getAttributes(PartialCompositeDirContext.java:121)
        at
com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.getAttributes(PartialCompositeDirContext.java:109)
        at
javax.naming.directory.InitialDirContext.getAttributes(InitialDirContext.java:121)
        at DNSLookupBug.main(DNSLookupBug.java:17)


3. bug description/possible fix

In com/sun/jndi/dns/DnsClient.java, 'xid' is of type 'int' but
initialized by the 'short' 'ident'. Thus if 'ident' reaches 32768, it
becomes negative, rendering 'xid' negative to '-32768':

~    private short ident = 0;           // used to set the msg ID field
[...]
        int xid;
        synchronized (identLock) {
            xid = ++ident;
        }

In the class com/sun/jndi/dns/Header.java, the answer from the DNS
server is converted backwards but to an 'int' instead of a 'short':

~   private static int getShort(byte[] msg, int pos) {
        return (((msg[pos] & 0xFF) << 8) |
                (msg[pos + 1] & 0xFF));
~    }

So, the result of this method gives '32768' which is not equal to
'-32768' and therefore the failure. A possible fix would be to replace

            xid = getShort(msg, pos);
by
            xid = (short) getShort(msg, pos);

in com/sun/jndi/dns/Header.java.


4. sample code

- ---------- BEGIN SOURCE ----------
import java.util.*;

import javax.naming.directory.*;

public class DNSLookupBug {
~    public static void main(String[] args) {
~        try {
~            final Hashtable env = new Hashtable();
~            env.put("java.naming.factory.initial",
~                    "com.sun.jndi.dns.DnsContextFactory");

~            DirContext dnsContext = new InitialDirContext(env);

~            final String[] domain = new String[] {"A"};
~            for (int i = 0; i < 0xffff; i++) {
~                dnsContext.getAttributes("sun.com", domain);
~                if (i % 1000 == 0) {
~                    // use this as a workaround
~                    //dnsContext = new InitialDirContext(env);
~                    System.out.println(i);
~                }
~            }

~        } catch (Exception e) {
~            e.printStackTrace();
~        }
~    }
}
- ---------- END SOURCE ----------


5. workaround

The application has to be rewritten to use a new InitialDirContext
before the limit of 32768 requests is reached. Another solution would be
to subclass InitialDirContext and let 'xid' have only positive values.
- --
Kurt Huwig             iKu Systemhaus AG        http://www.iku-ag.de/
Vorstand               Am Römerkastell 4        Telefon 0681/96751-0
~                       66121 Saarbrücken        Telefax 0681/96751-66
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