Microsoft DNS server unwitting DDoS contributor

[Pedro Luis Karrasquillo <peter_toyota () hotmail com>]

...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 () microsoft com 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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