"Code Red" Worm Exploiting Buffer Overflow In IIS Indexing Service DLL

Original release date: July 19, 2001
Last revised: January 17, 2002
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

  • 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 installed
  • Cisco CallManager, Unity Server, uOne, ICS7750, Building Broadband Service Manager (these systems run IIS)
  • Unpatched Cisco 600 series DSL routers


The CERT/CC has received reports of new self-propagating malicious code that exploits IIS-enabled systems susceptible to the vulnerability described in CERT advisory CA-2001-13 Buffer Overflow In IIS Indexing Service DLL. Other systems not directly vulnerable to this exploit may also be impacted. Reports indicate that two variants of the "Code Red" worm may have already affected more than 250,000 hosts.

A translation of this advisory into Polish is available at http://www.cert.pl/CA/CA-2001-19-PL.html.

I. Description

The "Code Red" worm is self-replicating malicious code that exploits a known vulnerability in Microsoft IIS servers (CA-2001-13).

Attack Cycle

The "Code Red" worm attack proceeds as follows:

  1. The "Code Red" worm attempts to connect to TCP port 80 on a randomly chosen host assuming that a web server will be found. Upon a successful connection to port 80, the attacking host sends a crafted HTTP GET request to the victim, attempting to exploit a buffer overflow in the Indexing Service described in CERT advisory CA-2001-13
  2. The same exploit (HTTP GET request) is sent to each of the randomly chosen hosts due to the self-propagating nature of the worm. However, depending on the configuration of the host which receives this request, there are varied consequences.
    • IIS 4.0 and 5.0 servers with Indexing service installed will almost certainly be compromised by the "Code Red" worm.
    • Unpatched Cisco 600-series DSL routers will process the HTTP request thereby triggering an unrelated vulnerability which causes the router to stop forwarding packets. [http://www.cisco.com/warp/public/707/cisco-code-red-worm-pub.shtml]
    • 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.
  3. If the exploit is successful, the worm begins executing on the victim host. In the earlier variant of the worm, victim hosts with a default language of English experienced the following defacement on all pages requested from the server:
    HELLO! Welcome to http://www.worm.com! Hacked By Chinese!
    Servers configured with a language that is not English and those infected with the later variant will not experience any change in the served content.

    Other worm activity on a compromised machine is time senstive; different activity occurs based on the date (day of the month) of the system clock.

    • Day 1 - 19: The infected host will attempt to connect to TCP port 80 of randomly chosen IP addresses in order to further propagate the worm.
    • Day 20 - 27: A packet-flooding denial of service attack will be launched against a particular fixed IP address
    • Day 28 - end of the month: The worm "sleeps"; no active connections or denial of service

System Footprint

The "Code Red" worm activity can be identified on a machine by the presence of the following string in a web server log files:


The presence of this string in a log file does not neccessarily indicate compromise. Rather it only implies that a "Code Red" worm attempted to infect the machine.

Additionally, web pages on victim machines may be defaced with the following message:

HELLO! Welcome to http://www.worm.com! Hacked By Chinese!

The text of this page is stored exclusively in memory and is not written to disk. Therefore, searching for the text of this page in the file system may not detect compromise.

Network Footprint

A host running an active instance of the "Code Red" worm scans random IP addresses on port 80/TCP looking for other hosts to infect.

Additional detailed analysis of this worm has been published by eEye Digital Security at http://www.eeye.com.

II. Impact

In addition to possible web site defacement, infected systems may experience performance degradation as a result of the scanning activity of this worm. This degradation can become quite severe since it is possible for a worm to infect a machine multiple times simultaneously.

Non-compromised systems and networks that are being scanned by other hosts infected by the "Code Red" worm may experience severe denial of service. In the earlier variant, this occurs because each instance of the "Code Red" worm uses the same random number generator seed to create the list of IP addresses it scans. Therefore, all hosts infected with the earlier variant scan the same IP addresses. This behavior is not found in the later variant, but the end result is the same due to the use of improved randomization techniques that facilitates more prolific scanning.

Furthermore, it is important to note that while the "Code Red" worm appears to merely deface web pages on affected systems and attack other systems, the IIS indexing vulnerability it 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 victim system.

III. Solutions

The CERT/CC encourages all Internet sites to review CERT advisory 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

Steps for Recovering from a UNIX or NT System Compromise

Since the worm resides entirely in memory, a reboot of the machine will purge it from the system. However, patching the system for the underlying vulnerability remains imperative since the likelihood of re-infection is quite high due to the rapid propagation of the worm.

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 comments.

Cisco Systems

Cisco has published a security advisory describing this vulnerability at


Microsoft Corporation

The following document regarding the vulnerability exploited by the "Code Red" worm is available from Microsoft:


Author(s): Roman Danyliw and Allen Householder

Copyright 2001 Carnegie Mellon University.

Revision History

Jul 19, 2001: Initial release
Jul 20, 2001: Multiple variants, vendor information 
Jul 30, 2001: Clarification of systems affected, attack cycle; addition of link to Polish translation 
Aug 16, 2001: Updated link to Microsoft cumulative patch
Aug 23, 2001: Updated contact information
Jan 17, 2002: Removed Reporting section, updated feedback link