Symantec Security Response - W32.Blaster.Worm © 1995-2003 Symantec Corporation. All rights reserved. Legal Notices Privacy Policy W32.Blaster.Worm Discovered on: August 11, 2003 Last Updated on: August 29, 2003 09:10:47 AM Based on the number of customer submissions and on information from Symantec's DeepSight Threat Management System, Symantec Security Response has upgraded this threat to a Category 4 from a Category 3 threat. W32.Blaster.Worm is a worm that exploits the DCOM RPC vulnerability (described in Microsoft Security Bulletin MS03-026) using TCP port 135. The worm targets only Windows 2000 and Windows XP machines. While Windows NT and Windows 2003 Server machines are vulnerable to the aforementioned exploit (if not properly patched), the worm is not coded to replicate to those systems. This worm attempts to download the msblast.exe file to the %WinDir%\system32 directory and then execute it. W32.Blaster.Worm does not have a mass-mailing functionality. Additional information and an alternate site from which to download the Microsoft patch is available in the Microsoft article, "What You Should Know About the Blaster Worm and Its Variants." We recommend that you block access to TCP port 4444 at the firewall level, and then block the following ports, if you do not use the following applications: TCP Port 135, "DCOM RPC" UDP Port 69, "TFTP" The worm also attempts to perform a Denial of Service (DoS) on the Microsoft Windows Update Web server (windowsupdate.com). This is an attempt to prevent you from applying a patch on your computer against the DCOM RPC vulnerability. Click here for more information on the vulnerability that this worm exploits, and to find out which Symantec products can help mitigate risks from this vulnerability. NOTE: Virus definitions will detect this threat having: Defs Version: 50811s Sequence Number: 24254 Extended Version: 8/11/2003, rev. 19 Symantec Security Response has developed a removal tool to clean the infections of W32.Blaster.Worm. W32.Blaster.Worm Webcast The following Webcast has been provided, which discusses the mitigation and remediation strategies, as well as provides a detailed description of the DoS attack: http://enterprisesecurity.symantec.com/content/webcastinfo.cfm?webcastid=63 Security Response has provided some information to aid network administrators in ongoing efforts to track down W32.Blaster.Worm infected machines on their respective network. Please see the document, "Detecting traffic due to RPC worms" for additional information. Also Known As: W32/Lovsan.worm.a [McAfee], Win32.Poza.A [CA], Lovsan [F-Secure], WORM_MSBLAST.A [Trend], W32/Blaster-A [Sophos], W32/Blaster [Panda], Worm.Win32.Lovesan [KAV] Type: Worm Infection Length: 6,176 bytes Systems Affected: Windows 2000, Windows XP Systems Not Affected: Linux, Macintosh, OS/2, UNIX, Windows 95, Windows 98, Windows Me, Windows NT CVE References: CAN-2003-0352 Virus Definitions (Intelligent Updater) * August 11, 2003 Virus Definitions (LiveUpdate™) ** August 11, 2003 *Intelligent Updater definitions are released daily, but require manual download and installation. Click here to download manually. **LiveUpdate virus definitions are usually released every Wednesday. Click here for instructions on using LiveUpdate. Wild: Number of infections: More than 1000 Number of sites: More than 10 Geographical distribution: High Threat containment: Moderate Removal: Moderate Threat Metrics Wild: HighDamage: MediumDistribution: High Damage Payload Trigger: If the date is the 16th of the month until the end of that month if it's before August, and every day from August 16 until December 31. Payload: Performs Denial of Service against windowsupdate.com Causes system instability: May cause machines to crash. Compromises security settings: Opens a hidden remote cmd.exe shell. Distribution Ports: TCP 135, TCP 4444, UDP 69 Target of infection: Machines with vulnerable DCOM RPC Services running. When W32.Blaster.Worm is executed, it does the following: Checks to see whether a computer is already infected and whether the worm is running. If so, the worm will not infect the computer a second time. Adds the value: "windows auto update"="msblast.exe" to the registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run so that the worm runs when you start Windows. Generates an IP address and attempts to infect the computer that has that address. The IP address is generated according to the following algorithms: For 40% of the time, the generated IP address is of the form A.B.C.0, where A and B are equal to the first two parts of the infected computer's IP address. C is also calculated by the third part of the infected system's IP address; however, for 40% of the time the worm checks whether C is greater than 20. If so, a random value less than 20 is subtracted from C. Once the IP address is calculated, the worm will attempt to find and exploit a computer with the IP address A.B.C.0. The worm will then increment the 0 part of the IP address by 1, attempting to find and exploit other computers based on the new IP address, until it reaches 254. With a probability of 60%, the generated IP address is completely random. Sends data on TCP port 135 that may exploit the DCOM RPC vulnerability. The worm sends one of two types of data: either to exploit Windows XP or Windows 2000. For 80% of the time, Windows XP data will be sent; and for 20% of the time, the Windows 2000 data will be sent. NOTES: The local subnet will become saturated with port 135 requests. While W32.Blaster.Worm cannot spread to the Windows NT or Windows Server 2003, unpatched computers running these operating systems may crash as a result of the worm's attempts to exploit them. However, if the worm is manually placed and executed on a computer running these operating systems, it can run and spread. Due to the random nature of how the worm constructs the exploit data, this may cause the RPC service to crash if it receives incorrect data. This may manifest as svchost.exe, generating errors as a result of the incorrect data. If the RPC service crashes, the default procedure under Windows XP and Windows Server 2003 is to restart the computer. To disable this feature, see step one of the Removal Instructions below. Uses Cmd.exe to create a hidden remote shell process that will listen on TCP port 4444, allowing an attacker to issue remote commands on an infected system. Listens on UDP port 69. When the worm receives a request from a computer to which it was able to connect using the DCOM RPC exploit, it will send msblast.exe to that computer and tell it to execute the worm. If the current date is the 16th through the end of the month for the months of January to August, or if the current month is September through December, the worm will attempt to perform a DoS on Windows Update. However, the attempt to perform the DoS will succeed only if one the following conditions is true: The worm runs on a Windows XP computer that was either infected or restarted during the payload period. The worm runs on a Windows 2000 computer that was infected during the payload period and has not been restarted since it was infected. The worm runs on a Windows 2000 computer that has been restarted since it was infected, during the payload period, and the currently logged in user is Administrator. The DoS traffic has the following characteristics: Is a SYN flood on port 80 of windowsupdate.com. Tries to send 50 HTTP packets every second. Each packet is 40 bytes in length. If the worm cannot find a DNS entry for windowsupdate.com, it uses a destination address of 255.255.255.255. Some fixed characteristics of the TCP and IP headers are: IP identification = 256 Time to Live = 128 Source IP address = a.b.x.y, where a.b are from the host ip and x.y are random. In some cases, a.b are random. Destination IP address = dns resolution of "windowsupdate.com" TCP Source port is between 1000 and 1999 TCP Destination port = 80 TCP Sequence number always has the two low bytes set to 0; the 2 high bytes are random. TCP Window size = 16384 The worm contains the following text, which is never displayed: I just want to say LOVE YOU SAN!! billy gates why do you make this possible ? Stop making money and fix your software!! Mitigating the DoS Payload As of August 15th, 2003, Microsoft has removed the DNS record for windowsupdate.com. While the DoS portion of the worm will not affect Microsoft's Windows Update feature, network administrators can use the following recommendations to mitigate the DoS payload: Reroute windowsupdate.com to a special internal IP address: This will alert you to the infected machines if you have a server listening to catch the SYN flood. Configure anti-spoofing rules on the routers, if this has not already been implemented: This will prevent a high percentage of packets from leaving the network. Using uRPF or egress ACLs would be effective. Symantec Client Security On August 15, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Blaster.Worm activity. Symantec Gateway Security On August 12, 2003, Symantec released an update for Symantec Gateway Security 1.0. Symantec's full application inspection firewall technology protects against this Microsoft vulnerability, blocking all the above listed TCP ports by default. For maximum security, third generation full application inspection technology intelligently blocks the tunneling of DCOM traffic over HTTP channels; thus, providing an extra layer of protection not readily available on most common network filtering firewalls. Symantec Host IDS On August 12, 2003, Symantec released an update for Symantec Host IDS 4.1. Intruder Alert On August 12, 2003, Symantec released Intruder Alert 3.6 W32_Blaster_Worm Policy. Symantec Enterprise Firewall Symantec's full application inspection firewall technology protects against the W32.Blaster.worm, blocking all the above listed TCP ports by default. Symantec ManHunt Symantec ManHunt Protocol Anomaly Detection technology detects the activity associated with this exploit as "Portsweep." Although ManHunt can detect activity associated with this exploit with the Protocol Anomaly Detection technology, you can use the "Microsoft DCOM RPC Buffer Overflow" custom signature, released in Security Update 4, to precisely identify the exploit being sent. Security Update 5 has been released to provide signatures specific to W32.Blaster.Worm, to include the detection of more attributes of W32.Blaster.Worm. Symantec ManHunt Protocol Anomaly Detection technology detects the activity associated with the DoS SYN flood. Security Response has created a custom signature for ManHunt 3.0, released in Security Update 6, to detect this attack specifically as a Blaster DDoS Request. Enterprise Security Manager Symantec Security Response posted a Response Policy for this vulnerability on July 17, 2003. Symantec Vulnerability Assessment Symantec Security Response posted a release that detects and reports the vulnerability on July 17, 2003. Symantec NetRecon Symantec NetRecon can identify machines that are susceptible to the W32.Blaster.Worm, by identifying the "Microsoft DCOM RPC Buffer Overflow" vulnerability. Refer to Symantec NetRecon SU6 for more details. Symantec Security Response offers these suggestions on how to configure Symantec products in order to minimize your exposure to this threat. Symantec Enterprise Firewall TCP ports 135 and 4444 and UDP port 69 are blocked by default. Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices": Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates. If a blended threat exploits one or more network services, disable, or block access to, those services until a patch is applied. Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services. Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised. Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files. Isolate infected computers quickly to prevent further compromising your organization. Perform a forensic analysis and restore the computers using trusted media. Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched. Removal using the W32.Blaster.Worm Removal Tool Symantec Security Response has developed a removal tool to clean the infections of W32.Blaster.Worm. This is the easiest way to remove this threat and should be tried first. Manual Removal As an alternative to using the removal tool, you can manually remove this threat. The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines. Restore Internet connectivity. End the worm process. Obtain the latest virus definitions. Scan for and delete the infected files. Reverse the changes made to the registry. Obtain the Microsoft HotFix to correct the DCOM RPC vulnerability. For specific details, refer to the following instructions: 1. Restoring Internet connectivity In many cases, on both Windows 2000 and XP, changing the settings for the Remote Procedure Call (RPC) service may allow you to connect to the Internet without the computer shutting down. To restore Internet connectivity to your PC, follow these steps: Click Start > Run. The Run dialog box appears. Type: SERVICES.MSC /S in the open line, and then click OK. The Services window opens. In the right pane, locate the Remote Procedure Call (RPC) service. CAUTION: There is also a service named Remote Procedure Call (RPC) Locator. Do not confuse the two. Right-click the Remote Procedure Call (RPC) service, and then click Properties. Click the Recovery tab. Using the drop-down lists, change First failure, Second failure, and Subsequent failures to "Restart the Service." Click Apply, and then OK. CAUTION: Make sure that you change these settings back once you have removed the worm. 2. Ending the Worm process Press Ctrl+Alt+Delete once. Click Task Manager. Click the Processes tab. Double-click the Image Name column header to alphabetically sort the processes. Scroll through the list and look for Msblast.exe. If you find the file, click it, and then click End Process. Exit the Task Manager. 3. Obtaining the latest virus definitions Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions: For newer computer users Running LiveUpdate, which is the easiest way to obtain virus definitions: Virus definitions for W32.Blaster.worm have been made available via the LiveUpdate server since August 11th, 2003. To obtain the latest virus definitions, click the LiveUpdate button from within the main user interface of your Symantec product. When running LiveUpdate, ensure that only "Norton AntiVirus Virus Definitions" are checked. Product updates can be obtained at a later time. For system administrators and advanced users Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater). The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions. 4. Scanning for and deleting the infected files Start your Symantec antivirus program and make sure that it is configured to scan all the files. For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files." For Symantec AntiVirus Enterprise products: Read the document, "How to verify that a Symantec corporate antivirus product is set to scan all files." Run a full system scan. If any files are detected as infected with W32.Blaster.Worm, click Delete. 5. Reversing the changes made to the registry CAUTION: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry," for instructions. Click Start, and then click Run. (The Run dialog box appears.) Type regedit Then click OK. (The Registry Editor opens.) Navigate to the key: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run In the right pane, delete the value: windows auto update Exit the Registry Editor. 6. Obtaining the Microsoft HotFix to correct the DCOM RPC vulnerability W32.Blaster.Worm is a worm that exploits the DCOM RPC vulnerability using TCP port 135 to infect your PC. The W32.Blaster.Worm also attempts to perform a DoS on the Microsoft Windows Update Web server (windowsupdate.com) using your PC. To fix this, it is important to obtain the Microsoft Hotfix at: Microsoft Security Bulletin MS03-026. Additional information: Additional information and an alternate site from which to download the Microsoft patch is available in the Microsoft article, "What You Should Know About the Blaster Worm and Its Variants." Revision History: August 28, 2003: Added link to document with information about Blaster related network traffic. August 20, 2003: Added reference to Symantec Client Security. August 15, 2003: Added additional recommendation pertaining to mitigating the DoS attack. Added reference to the updates for the Symantec NetRecon and Symantec Vulnerability Assessment. Added link to Symantec Webcast. Additional information about Symantec ManHunt updates. August 14, 2003: Provided recommendations for mitigating the DoS attack. Updated DoS payload information. Added information about the DoS traffic. August 13, 2003: Re-ordered major steps in removal instructions. Added the download location. Minor formatting updates. Removed Windows system restore instructions from removal August 12, 2003: Upgraded to Category 4 from Category 3, based on increased rate of submissions. Added additional aliases. Updated the Technical Description section. Added information to the Removal on changing the settings for RPC. Write-up by: Douglas Knowles, Frederic Perriot and Peter Szor