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Microsoft Windows (all versions) II
Week of July 27, 2001
CERT Advisory CA-2001-23 Continued Threat of the 'Code Red' Worm
Original release date: July 26, 2001
A complete revision history can be found at the end of this file.
* Microsoft Windows NT 4.0 with IIS 4.0 or IIS 5.0 enabled and Index
Server 2.0 installed
* Windows 2000 with IIS 4.0 or IIS 5.0 enabled and Indexing services
* Cisco CallManager, Unity Server, uOne, ICS7750, Building Broadband
Service Manager (these systems run IIS)
* Unpatched Cisco 600 series DSL routers
Since around July 13, 2001, at least two variants of the
self-propagating malicious code 'Code Red' have been attacking hosts
on the Internet (see CA-2001-19 'Code Red' Worm Exploiting Buffer
Overflow In IIS Indexing Service DLL). Different organizations who
have analyzed 'Code Red' have reached different conclusions about the
behavior of infected machines when their system clocks roll over to
the next month. We believe the worm will begin propagating again on
August 1, 2001 0:00 GMT, and there is evidence that tens of thousands
of systems are already infected or vulnerable to re-infection at that
time. Because the worm propagates very quickly, it is likely that
nearly all vulnerable systems will be compromised by August 2, 2001.
The CERT/CC has received reports indicating that at least 280,000
hosts were compromised in the first wave.
The 'Code Red' worm is malicious self-propagating code that exploits
Microsoft Internet Information Server (IIS)-enabled systems
susceptible to the vulnerability described in CA-2001-13 Buffer
Overflow In IIS Indexing Service DLL. Its activity on a compromised
machine is time senstive; different activity occurs based on the date
(day of the month) of the system clock. The CERT/CC is aware of at
least two major variants of the worm, each of which exhibits the
following pattern of behavior:
* Propagation mode (from the 1st - 19th of the month): The infected
host will attempt to connect to TCP port 80 of randomly chosen IP
addresses in order to further propagate the worm. Depending on the
configuration of the host that receives this request, there are
+ Unpatched IIS 4.0 and 5.0 servers with Indexing service
installed will almost certainly be compromised by the 'Code
Red' worm. In the earlier variant of the worm, victim hosts
with a default language of English experienced a defacement
on all pages requested from the web server. Hosts infected
with the later variant did not experience any change in the
+ Unpatched Cisco 600-series DSL routers will process the HTTP
request and trigger an unrelated vulnerability that causes
the router to stop forwarding packets.
+ Systems not running IIS, but with an HTTP server listening on
TCP port 80 will probably accept the HTTP request, return
with an 'HTTP 400 Bad Request' message, and potentially log
this request in an access log.
* Flood mode (from the 20th - 27th of the month): A packet-flooding
denial-of-service attack will be launched against a specific IP
address embedded in the code.
* Termination (after the 27th day): The worm remains in memory but
is otherwise inactive.
Detailed technical analysis of the 'Code Red' worm can be found in
Data reported to the CERT/CC indicates that the 'Code Red' worm
infected more than 250,000 sytems in just 9 hours. Figure 1
illustrates the activity between 6:00 AM EDT and 8:00 PM EDT on July
[See Figure 1 at http://www.cert.org/advisories/CA-2001-23.html]
NOTE: After 8:00 PM EDT on July 19 (0:00 GMT July 20), the worm
switched into flood mode on most infected systems, so the number of
infected systems remained fairly constant after that time.
Our analysis estimates that starting with a single infected host, the
time required to infect all vulnerable IIS servers with this worm
could be less than 18 hours. Since the worm is programmed to continue
propagating for the first 19 days of the month, widespread denial of
service may result due to heavy scan traffic.
As reported in CA-2001-19, infected systems may experience web site
defacement as well as performance degradation as a result of the
propagating activity of this worm. This degradation can become quite
severe, and in fact may cause some services to stop entirely, since it
is possible for a machine to be infected with multiple copies of the
Furthermore, it is important to note that the IIS indexing
vulnerability that the 'Code Red' worm exploits can be used to execute
arbitrary code in the Local System security context. This level of
privilege effectively gives an attacker complete control of the
The CERT/CC encourages all Internet sites to review CA-2001-13 and
ensure workarounds or patches have been applied on all affected hosts
on your network.
If you believe a host under your control has been compromised, you may
wish to refer to
Known versions of the worm reside entirely in memory; therefore, a
reboot of the machine will purge the worm from the system. However,
due to the rapid propagation of the worm, the likelihood of
re-infection is quite high. Taking the system offline and applying the
vendor patch will eliminate the vulnerability exploited by the 'Code
IV. Good Practices
Consistent with the security best-practice of denying all network
traffic and only selectively allowing that which is required, ingress
and egress filtering should be implemented at the network edge.
Likewise, controls must be in place to ensure that all software used
on a network is properly maintained.
Ingress filtering manages the flow of traffic as it enters a network
under your administrative control. Servers are typically the only
machines that need to accept inbound connections from the public
Internet. In the network usage policy of many sites, there are few
reasons for external hosts to initiate inbound connections to machines
that provide no public services. Thus, ingress filtering should be
performed at the border to prohibit externally initiated inbound
connections to non-authortized services. In this fashion, the
effectiveness of many intruder scanning techniques can be dramatically
reduced. With 'Code Red,' ingress filtering will prevent instances of
the worm outside of your network from infecting machines in the local
network that are not explicitly authorized to provide public web
Egress filtering manages the flow of traffic as it leaves a network
under your administrative control. There is typically limited need for
machines providing public services to initiate outbound connections to
the Internet. In the case of 'Code Red,' employing egress filtering
will prevent compromised IIS servers on your network from further
propagating the worm.
Installing new software with the latest patches
When installing an operating system or application on a host for the
first time, it is insufficient to merely use the install media.
Vulnerabilities are often discovered after the software becomes widely
distributed. Thus, prior to connecting this host to the network, the
latest security patches for the software should be obtained from the
vendor and applied.
Appendix A. - Vendor Information
This appendix contains information provided by vendors for this
advisory. When vendors report new information to the CERT/CC, we
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their
Cisco has published a security advisory describing this vulnerability
The following document regarding the vulnerability exploited by the
'Code Red' worm is available from Microsoft:
Author(s): Roman Danyliw and Allen Householder
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4)
Monday through Friday; they are on call for emergencies during other
hours, on U.S. holidays, and on weekends.
We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more
Getting security information
CERT publications and other security information are available from
our web site
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Copyright 2001 Carnegie Mellon University.
Jul 26, 2001: Initial release