1. Symantec/
  2. Security Response/
  3. Trojan.Vundo


Risk Level 2: Low

November 20, 2004
August 9, 2012 2:30:01 PM
Infection Length:
Systems Affected:
1. Prevention and avoidance
1.1 User behavior and precautions
1.2 Operating system and software patches
2. Infection method
2.1 Email
2.2 Other malware
2.3 Peer-to-peer file-sharing
2.4 Drive-by downloads
2.5 Fake codecs
3. Functionality
3.1 Display of advertisements
3.2 Pop-up and pop-under advertising
3.3 JavaScript injection
3.4 Modification of search results
3.5 Web browser monitoring
3.6 Disabling of system and security software
3.7 User interface changes
3.8 System modifications
3.9 Network activity
4. Additional information


The following actions can be taken to avoid or minimize the risk from this threat.

1.1 User behavior and precautions
Users should be aware that email messages with malicious content may appear to have been sent by people known to them, and as such the fact that the sender is known does not guarantee the safety of any particular message.

Spam emails may contain malicious links that have been disguised or otherwise made to appear benign. Users should exercise caution when following links in email messages, especially if:
  • The sender is not known, or
  • Given the sender, the characteristics of the email are unusual, or
  • The link is to an unknown domain or an executable file

Users should avoid opening email attachments unless their authenticity can be verified.

The downloading of files via peer-to-peer file-sharing networks can lead to infection. Users should avoid downloading files from unknown or untrusted sources, including fake video Web sites that may serve the Trojan executable under guise of it being a codec that is required to watch a streaming video.

1.2 Operating system and software patches
Users are advised to ensure that their operating systems and installed software are fully patched, and that antivirus and firewall software is up to date and operational. Users should turn on automatic updates if available so that their computers can receive the latest patches and updates when they are distributed by software vendors.

This threat is known to infect computers through a number of methods. Each of these methods is examined in more detail below.

2.1 Email
As with most spam email, the campaigns that spread Trojan.Vundo generally attempt to entice or coerce the user into running an attached file, or clicking on a link to a malicious file. A malicious URL may also lead to a browser exploit that results in the malicious file being executed. This may be done by crafting file names or URLs to appear safe or to resemble those from sources trusted by the user.

Malicious executable files may be attached to emails with innocuous or enticing names, be compressed within .zip files, and may also have file names that have been specially crafted in order to make the executable appear to be a different type of file.

2.2 Other malware
As Trojan.Vundo allows attackers to generate revenue from compromised computers, it is often downloaded by other malware. The Ackantta family of mass-mailing worms – including W32.Ackantta.B@mm and W32.Ackantta.C@mm – typically downloads Trojan.Vundo on to compromised computers using HTTP and therefore significantly contributes to infection levels of the threat.

2.3 Peer-to-peer file-sharing
Trojan.Vundo spreads via peer-to-peer file sharing networks. The Trojan may deliberately be shared by attackers seeking to increase the infection levels of the threat, and as such may be given an enticing name in order to tempt users into downloading the malicious executable. Typical enticing names include those of otherwise expensive commercial software packages, key generators, and 'cracked' versions of high-end applications. Copies of the threat masquerading as adult pictures and video files are also common, especially those that include the names of celebrities intended to pique users' interest.

2.4 Drive-by downloads
Trojan.Vundo is known to be spread by Web sites that exploit known vulnerabilities in Web browsers and their associated plugins. These exploits are often served by commercially available exploit kits and as such need not necessarily be crafted by individuals with a high degree of technical ability. This also means that the vulnerabilities chosen to be exploited change frequently and according to ease of exploitation.

2.5 Fake codecs
Web sites that purport to host streaming videos may be used to distribute copies of the Trojan. An executable masquerading as a required codec is downloaded when a user attempts to watch the deliberately non-functional embedded video. Search engine poisoning may be used to increase the likelihood of users finding the fraudulent site when using search engines; particularly newsworthy or currently trending topics are often chosen by attackers, seemingly without any moral concern.


Trojan.Vundo consists of three main DLLs:
  • A DLL to download files
  • A DLL to lower security settings
  • A DLL to monitor Web browsing and display advertisements (i.e. the Trojan’s payload)

Each of these DLLs is capable of functioning independently on the compromised computer.

The first Trojan.Vundo component, the downloader DLL, typically arrives on the computer by way of an initial dropper executable. When executed, the downloader component then downloads the other two component DLLs on to the compromised computer, which it then executes. The DLL containing the Trojan’s payload is then injected into all running processes, including Web browsers.

Depending on the variant, the downloader DLL component may also download other files, such as misleading applications or copies of other malware.

3.1 Display of advertisements
The primary aim of Trojan.Vundo is to display advertisements on the compromised computer. This aim is accomplished through several different methods:
  • Displaying pop-up and pop-under advertisements
  • Injecting JavaScript into HTML pages viewed on the compromised computer
  • Altering links returned by search engines

3.2 Pop-up and pop-under advertising
The Trojan displays both pop-up and pop-under advertising, typically for fraudulent or misleading applications and adult Web sites.

Clicking the pop-ups results in further fake scans. Web browsers may also be redirected to potentially malicious Web sites.

3.3 JavaScript injection
Along with the main pop-up advertising functionality detailed above, Trojan.Vundo also injects JavaScript into the HTML of search result pages returned by the following search engines:
  • AltaVista
  • AOL Search
  • Ask
  • Bing
  • FastSearch
  • Google
  • Hotbot
  • Live
  • Lycos
  • Yahoo

The JavaScript injected by the Trojan causes further pop-up advertisements to be displayed on the compromised computer.

3.4 Modification of search results
Trojan.Vundo is additionally able to modify search results returned by the search engines mentioned above. The Trojan analyzes the HTML of search result pages and inserts its own HTML links in place of the links in the original pages. This causes users to be redirected to sites they would otherwise not have accessed. The pages to which users may be redirected include adult and pornographic sites, sites advertising misleading applications, fake security scans, and pages advertising various other products and services.

3.5 Web browser monitoring
Trojan.Vundo monitors Web browsing on the compromised computer and relays the URLs visited to a remote location using HTTP post operations. It also sends other system information, including the following:
  • System information
  • Affiliate IDs for advertising and pay-per-install software
  • Browser name

Note: The HTTP post data is encoded using base64.

3.6 Disabling of system and security software
In order to impede manual removal and prolong its presence on the compromised computer, Trojan.Vundo may create or modify certain registry entries to disable system tools that may be used to rid the computer of the threat. The modifications include (but are not limited to) the following:
  • Disabling the Task Manager, Registry Editor, and the Microsoft System Configuration Utility
  • Preventing Windows Automatic Updates
  • Altering Safe Mode settings
  • Disabling antivirus software, including Windows Defender
  • Blocking antivirus software updates

3.7 User interface changes
The Trojan may alter Explorer settings that affect the way in which the user is able to interact with the compromised computer. It may, for example, alter the Desktop wallpaper, the screensaver, and the items that are displayed in the Windows Control Panel Display settings.

3.8 System modifications

Files/folders created
Trojan.Vundo consists of three DLLs named randomly as follows:


The DLLs that comprise the Trojan have been observed in the following folders:
  • %Windir%
  • %Windir%\addins
  • %Windir%\AppPatch
  • %Windir%\assembly
  • %Windir%\Config
  • %Windir%\Cursors
  • %Windir%\Driver Cache
  • %Windir%\Drivers
  • %Windir%\Fonts
  • %Windir%\Help
  • %Windir%\inf
  • %Windir%\java
  • %Windir%\Microsoft
  • %Windir%\Microsoft.NET
  • %Windir%\msagent
  • %Windir%\Registration
  • %Windir%\repair
  • %Windir%\security
  • %Windir%\ServicePackFiles
  • %Windir%\Speech
  • %Windir%\system
  • %Windir%\system32
  • %Windir%\Tasks
  • %Windir%\Web
  • %Windir%\Windows Update Setup Files

Files/folders deleted

Files/folders modified

Registry subkeys/entries created
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\SharedTaskScheduler\"[RANDOM CLSID]" = "[NINE CHARACTERS]"
  • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run\"[NINE CHARACTERS]" = "Rundll32.exe "[PATH TO THREAT]\[RANDOM CHARACTERS].dll",a."
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\ShellServiceObjectDelayLoad\"[NINE CHARACTERS]" = "[RANDOM CLSID]"

Note: [NINE CHARACTERS] is generated from the serial number of %SystemDrive%.

Registry subkeys/entries deleted

Registry subkeys/entries modified
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\"AppInit_DLLs" = "[PATH TO THREAT]\[RANDOM CHARACTERS].dll"
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\"LoadAppInit_DLLs" = "4"

Note: [NINE CHARACTERS] is generated from the serial number of %SystemDrive%.

3.9 Network activity
The threat may perform the following network activities.

The Trojan may download and execute additional malicious executables.


Other network activity
Trojan.Vundo monitors Web browsing on the compromised computer. Each time a URL is visited, the Trojan performs a base-64-encoded HTTP post operation that contains the following information:
  • System information
  • Affiliate IDs for advertising and pay-per-install software
  • Browser name
  • URL visited

This information allows the threat to target the advertisements more specifically to the user.

For more information relating to this threat family, please see the following resources:


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • 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.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a 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.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. 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.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Henry Bell and Eric Chien
Summary| Technical Details| Removal

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