[CORE-2003-12-05] DCE RPC Vulnerabilities New Attack Vectors Analysis

                   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.

[email protected]
[email protected]

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
#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()
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 $