Core Security Technologies Advisory
                           http://www.coresecurity.com

               DCE RPC Vulnerabilities New Attack Vectors Analysis



Date Published: 2003-12-10

Last Update: 2003-12-10

Advisory ID: CORE-2003-12-05

Title: DCE RPC Vulnerabilities New Attack Vectors Analysis

Remotely Exploitable: Yes

Locally Exploitable: Yes

Advisory URL:
  http://www.coresecurity.com/common/showdoc.php?idx=393&idxseccion=10

Vendors contacted:
  - Microsoft
    . Core notification: 2003-12-09
    . Notification acknowledged by Microsoft: 2003-12-09

Release Mode: USER RELEASE

*Vulnerability Description:*

  Core Security Technologies researchers discovered new attack vectors for
  recently published vulnerabilities in Microsoft Windows operating systems.

  These new attack methods were found while researching exploitation conditions
  for the Workstation Service vulnerability discovered by eEye Digital Security
  and disclosed in Microsoft security bulletin MS03-049 of November 11th, 2003.

  They might also apply to other vulnerabilities such as the DCE RPC DCOM
  and the Messenger service vulnerabilities addressed by bulletins MS03-001,
  MS03-026 and MS03-043.

  We found that by combining three protocol characteristics common to the
  vulnerabilities mentioned, an attacker can devise more severe, stealthy and
  low-noise attack vectors than those originally concieved. This creates the
  opportunity for malicious software to compromise large numbers of vulnerable
  systems in a massive scale, much like the Blaster and Slammer worms that caused
  great damage earlier in 2003.

  Core Security Technologies urges users of Microsoft Windows operating systems
  to deploy the available patches for these vulnerabilities as they effectively
  fix the problem. Suggested workarounds should be revisited to ensure that they
  address all currently known attack vectors properly (including the new ones
  disclosed in this advisory).

*Vulnerable Packages:*

  Microsoft RPC services running on Windows 2000 and Windows XP.

*Solution/Vendor Information/Workaround:*

  Patches are readily available to fix the vulnerabilities and close all known
  attack vectors.

  See Microsoft Security Bulletins [MS03-001], [MS03-026], [MS03-043],[MS03-049]:

  http://www.microsoft.com/technet/security/bulletin/MS03-001.asp
  http://www.microsoft.com/technet/security/bulletin/MS03-026.asp
  http://www.microsoft.com/technet/security/bulletin/MS03-043.asp
  http://www.microsoft.com/technet/security/bulletin/MS03-049.asp


*Credits:*

  This advisory was researched and prepared by Javier Kohen and Juliano
  Rizzo from Core Security Technologies.

  jkohen@coresecurity.com
  juliano@coresecurity.com

  The vulnerabilities mentioned herein where discovered independently by
  David Litchfield of Next Generation Security Software, the Last Stage
  of Delirum (LSD) Research Group, and Yuji Ukay, Barnaby Jack and
  Riley Hasell of eEye Digital Security.

*Technical Description - Exploit/Concept Code:*

  In recent months, several vulnerabilities in the Microsoft RPC code
  (see [MS03-001], [MS03-026], [MS03-043], [MS03-049]) have been disclosed.

  The RPC vulnerabilities account started back in July when LSD disclosed a
  severe security hole in the DCOM service. Since then, different workarounds
  were discussed on several security mailing lists but doubt persisted as to
  which RPC protocol sequences were potential attack gates. This is, obviously,
  an important factor in determining which ports should be filtered to prevent
  remote attacks and how other workarounds should be deployed.

  We have researched three protocol characteristics which when used together
  provide the attacker with new severe, stealthy and low-noise attack vectors.

  We were able to successfully exploit some of the latest DCE RPC vulnerabilities
  through less noted ports and even on broadcast addresses.

  The following sections provide more specific details about these attack
  vectors.

  [Some RPC services listen on high ports.]

  This is a little-known feature that has been omitted from the published alerts
  to date. The  importance of this issue lies in the fact that the most common
  filtering rules used on current firewall configurations will allow incoming
  traffic on these ports.

  For instance, the latest Workstation Service vulnerability can be exploited on
  a high TCP/UDP port. Usually the TCP port is 1025 and the UDP port is higher,
  depending on system settings.

  [Some RPC services listen to broadcast traffic.]

  Further tests showed that those attacks conducted with datagram protocols
  (like UDP) could be targeted to broadcast addresses and still succeed.
  Our first tests required performing a two-way handshake with each host
  that responded to the broadcasted query. This situation seems to confuse
  the native Windows 2000 RPC implementation, so we suspected that this
  kind of attack can not be built with the stock implementation.

  [The idempotent bit.]

  RPC has an interesting feature that allows the client to avoid the
  two-way handshake customary to datagram protocols. This can be enabled
  by turning on the idempotent flag in RPCv4 request packets. This not
  only reduces the traffic needed to perform the attack, but it also makes it
  possible to spoof the request's source. This handshake involves a 20-byte
  secret number, apparently not easily guessable, that can be avoided by
  setting the idempotent flag.

  We were able to exploit [MS03-026] using 445/TCP 139/TCP 135/TCP 135/UDP
  and 80/TCP. [MS03-049] can be successfully exploited through 445/TCP
  139/TCP and dynamically assigned TCP/UDP ports over 1024. We have not seen
  public exploits or worms using those ports, and we are not sure whether the
  Windows API can be bent for this purpose. We used our own DCE RPC
  implementation which is part of the publicly available Impacket
  project. Presumably [MS03-043] (Messenger service) can be exploited
  using the same techniques, but we haven't attempted an attack, although
  third party reports describe messenger service attacks using UDP broadcasts
  in the wild.


  [Firewall bypassing.]

  Since the attack can be conducted over the UDP
  protocol and that it can be spoofed, it is easy to bypass common
  filtering rules. Some personal firewalls enable the blocking of traffic on an
  application basis, but some of the vulnerable services actually run
  inside the same application that does the DNS resolution. This can be
  used to the attacker's advantage to reach the vulnerable targets by spoofing
  the attack packets as if they came from a legitimate server sending
  DNS responses back to DNS clients on vulnerable workstations.

  It is common to see filtering rules like the following:

  allow UDP packets from DNSSERVER port 53 to WORKSTATION port above 1024

  The outlined attack vector will pass through the above rule and succeed.
  Even personal firewall rules that specify an application will allow these
  attacks to pass:

  allow UDP packets from DNSSERVER port 53 to WORKSTATION application
  services.exe


  [Conclusions.]

  Patches for the vulnerabilities mentioned have already been made available
  by Microsoft. Installing them will effectively fix the bugs and close all
  attack vectors discovered herein.
  Workarounds should be revisited to ensure they properly cover these
  attack vectors.
  As a general conclusion, we recommend careful inspection of Windows Service
  vulnerabilities in order to identify potential avenues of attack related to
  these services providing RPC endpoints that listen to UDP and TCP traffic
  on high ports.

  [Appendix.]

  [DCE RPC protocol sequences.]

  A protocol sequence is a "character string that represents a valid
  combination of an RPC protocol (such as ncacn), a transport protocol
  (such as TCP), and a network protocol (such as IP)" (see [MSDN]).

  [What protocol sequences are available?]

  A standard Windows installation has default services accessible
  through many protocol sequences. For example, the Workstation Service
  can be accessed by means of the following:

  ncacn_ip_tcp:[####]
  ncacn_np:[\\PIPE\wkssvc]
  ncadg_ip_udp:[####]

  Notes: #### is a port number above 1024. Datagram based sequences
  (like ncadg_ip_udp) are also accessible through the broadcast address.
  Named pipes (strings like ncacn_np) can be contacted in several ways, via
  TCP ports 139, 445, 593 and 80.


  [How easy is it to build an attack over an alternative transport?]

  Starting from a working attack to an RPC service it is trivial to adapt
  it to work over other protocol sequences. Of course the attacker must
  have a DCE RPC implementation that allows her to use her choice of
  transport, here is where Impacket fits perfectly into the task as
  changing transports requires no additional effort.

===IMPACKET EXAMPLE=======

class ExploitPacket(ImpactPacket.Header):
      OP_NUM = 0x1B # Interface's method number
      def get_header_size(self):
          return 0
      def __init__(self, aBuffer = None):
          ImpactPacket.Header.__init__(self, 0)

class Attack:
      def do(self,host):
          # The next two lines could be changed to use different
   # protocol sequences.
          #port = 445
   # SMB over IP/TCP on port 445
          #stringbinding = "ncacn_np:%s[\\pipe\\service_np]" % host
          port = 135
   # IP/TCP transport on the specified port
          stringbinding = "ncacn_ip_tcp:%s[%d]" % (host,port)
          exploitStub = ImpactPacket.Data()
          exploitStub.set_bytes_from_string(exploitBytes)
          exploitPacket = ExploitPacket()
          exploitPacket.contains(exploitStub)
          rpcTransport = transport.DCERpcTransportFactory(stringbinding)
          rpcTransport.set_dport(port)
          # Uncomment for UDP protocols:
          #rpcConn = impacket.dcerpc.dcerpc_v4.DCERPC_v4(rpcTransport)
          rpcConn = impacket.dcerpc.dcerpc.DCERPC_v5(rpcTransport)
          rpcConn.connect()
          rpcConn.bind(SERVICE_UUID) # 20-byte UUID (including version)
          rpcConn.send(exploitPacket)
          #( ... exploit specific code ...)
          rpcConn.disconnect()

==========================

  [Retrieving the list of RPC endpoints.]

  Core Security Technologies provides, as part of its free, open source
  Impacket package (downloadable from http://oss.coresecurity.com/ ),
  a tool that allows remote enumeration of RPC of services listening
  and their assigned port numbers and supported transports.

  This code is platform independent Python.  A similar tool (RPCDUMP)
  is available from Microsoft.

*References:*

  [Core Security Technologies - Impacket tools]
  http://oss.coresecurity.com/

  [Microsoft]

  MS03-001
  http://www.microsoft.com/technet/security/bulletin/MS03-001.asp

  MS03-026
  http://www.microsoft.com/technet/security/bulletin/MS03-026.asp

  MS03-043
  http://www.microsoft.com/technet/security/bulletin/MS03-043.asp

  MS03-049
  http://www.microsoft.com/technet/security/bulletin/MS03-049.asp

  MSDN reference to endpoints and protocol sequences
  http://msdn.microsoft.com/library/en-us/midl/midl/endpoint.asp

  [eEye Digital Security]

  Windows Workstation Service Remote Buffer Overflow
  http://www.eeye.com/html/Research/Advisories/AD20031111.html

  Microsoft RPC Heap Corruption Vulnerability - Part II
  http://www.eeye.com/html/Research/Advisories/AD20030910.html

  [Last Stage of Delirum Research Group]
  http://lsd-pl.net/files/get?WINDOWS/win32_dcom

  [Next Generation SEcurity Software]
  http://www.nextgenss.com/research.html

*About Core Security Technologies*

  Core Security Technologies develops strategic security solutions for
  Fortune 1000 corporations, government agencies and military
  organizations. The company offers information security software and
  services designed to assess risk and protect and manage information
  assets.
  Headquartered in Boston, MA, Core Security Technologies can be reached
  at 617-399-6980 or on the Web at http://www.coresecurity.com.

  To learn more about CORE IMPACT, the first comprehensive penetration
  testing framework, visit:
  http://www.coresecurity.com/products/coreimpact


*DISCLAIMER:*

  The contents of this advisory are copyright (c) 2003 CORE Security
  Technologies and may be distributed freely provided that no fee is
  charged for this distribution and proper credit is given.

$Id: DCE\040RPC\040new\040attack\040vectors.txt,v 1.1 2003/12/10
20:21:16 carlos Exp $