[NT] DCE RPC Vulnerabilities New Attack Vectors Analysis
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To: firstname.lastname@example.org Date: 14 Dec 2003 14:05:51 +0200
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DCE RPC Vulnerabilities New Attack Vectors Analysis
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 11, 2003.
They might also apply to other vulnerabilities such as the DCE RPC DCOM
and the Messenger service vulnerabilities addresses 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).
* Microsoft RPC services running on Windows 2000 and Windows XP
Patches are readily available to fix the vulnerabilities and close all
known attack vectors.
See Microsoft Security Bulletins [MS03-001], [MS03-026], [MS03-043],
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
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
vulnerabilities through less noted ports and even on broadcast addresses.
The following sections provide more specific details about these attack
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 could 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 that is part of the publicly available Impacket project.
Presumably, [MS03-043] (Messenger service) can be exploited using the same
techniques, but we have not attempted an attack, although third party
reports describe messenger service attacks using UDP broadcasts in the
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
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
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.
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.
OP_NUM = 0x1B # Interface's method number
def __init__(self, aBuffer = None):
# The next two lines could be changed to use different protocol
#port = 445
#stringbinding = "ncacn_np:%s[\\pipe\\wkssvc]" % 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/>
http://oss.coresecurity.com/), a tool that allows remote enumeration of
RPC of services listening and their assigned port numbers and supported
This code is platform independent Python. A similar tool (RPCDUMP) is
available from Microsoft.
The advisory can be found at:
The information has been provided by <mailto:email@example.com>
Core Security Technologies.
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