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jreDoS.txt

jreDoS.txt
Posted Nov 10, 2004
Authored by Kurt Huwig

Java Runtime Environment versions 1.4.2, 1.5.0, and possible others are susceptible to a remote denial of service attack.

tags | advisory, java, remote, denial of service
SHA-256 | 0834f0d8c97e493e37382431176c3e2b024f4b9c11cb1d4bb30104ea6d9bcaef

jreDoS.txt

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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 R├Âmerkastell 4 Telefon 0681/96751-0
~ 66121 Saarbr├╝cken Telefax 0681/96751-66
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