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[Full-disclosure] Microsoft DNS server unwitting DDoS contributor

...not sure how the first message got included in the freepbx thread, so I am 
trying again:

Microsoft has responded to my report to secure@xxxxxxxxxxxxx and I can now 
disclose what I found. Microsoft determined this is all by design, so no 
changes to the server functionality is planned.

There is a minor bug on the MS Server 2008 DNS service that responds with the 
list of all root servers when queried for non-authoritative domains, EVEN when 
recursion is set to OFF. This allows a malicious party to spoof the source ip 
on a udp DNS request to any MS Server 2008 DNS and elicit a 533 byte response 
to a victim, making the server a contributor to coordinated Distributed Denial 
of Service attacks. The response contains the default list of root DNS servers.
Version tested: MS DNS on MS Server 2008 R2 version 6.1.7601.17514

Server is Authoritative to only one .com domain.

Config Parameters:

    DNS Recursion set to "disable"
    Enable Round Robin
    Enable Netmask Ordering
    Secure Cache against pollution 

And My Mitigation steps:

    Remove all root DNS servers listed on the "Root Hints" tab. 

This will not negatively affect the DNS functionality of the server when 
deployed only as an authoritative server for a specific domain.


Although RFC1034 on page 21 does allow the DNS to reply with the list of root 
servers (if configured) as a response option, ultimately it is preferable for 
it to mimic the behavior of BIND and not respond at all under these test 
conditions, to discourage abuse from malicious entities.


More details with images and packet captures and the Microsoft responses, in my 
web file http://pe.lúka.com/


Pedro
CCNP, CCDA, CCNA-Security, SANS GPEN
...But mostly a curious guy.
                                          
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