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[CORE-2003-12-05] DCE RPC Vulnerabilities New Attack Vectors Analysis
From: Core Security Technologies <advisories () coresecurity com>
Date: Thu, 11 Dec 2003 12:34:42 -0300

                      Core Security Technologies Advisory

              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:

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
 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
 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
 attack vectors.

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



 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
 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

 We were able to successfully exploit some of the latest DCE RPC
 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
 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
 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
 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
 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
 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


 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
 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.


 [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:


 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.


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
         #port = 445
         #stringbinding = "ncacn_np:%s[\\pipe\\service_np]" % host # SMB
over IP/TCP on port 445
         port = 135
         stringbinding = "ncacn_ip_tcp:%s[%d]" % (host,port) # IP/TCP
transport on the specified port
         exploitStub = ImpactPacket.Data()
         exploitPacket = ExploitPacket()
         rpcTransport = transport.DCERpcTransportFactory(stringbinding)
         # Uncomment for UDP protocols:
         #rpcConn = impacket.dcerpc.dcerpc_v4.DCERPC_v4(rpcTransport)
         rpcConn = impacket.dcerpc.dcerpc.DCERPC_v5(rpcTransport)
         rpcConn.bind(SERVICE_UUID) # 20-byte UUID (including version)
         #( ... exploit specific code ...)


 [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.


 [Core Security Technologies - Impacket tools]






 MSDN reference to endpoints and protocol sequences

 [eEye Digital Security]

 Windows Workstation Service Remote Buffer Overflow

 Microsoft RPC Heap Corruption Vulnerability - Part II

 [Last Stage of Delirum Research Group]

 [Next Generation SEcurity Software]

*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
 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:


 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 $


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