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


Risk Level 2: Low

July 13, 2011
November 29, 2013 11:16:11 AM
Infection Length:
Systems Affected:
CVE References:
CVE-2006-0003, CVE-2008-2992, CVE-2009-0927, CVE-2009-1671, CVE-2009-1672, CVE-2009-4324, CVE-2010-1885
1. Prevention and avoidance
1.1 User behavior and precautions
1.2 Patch operating system and software
1.3 Address blocking
2. Infection method
3. Functionality
3.1 System modifications
3.2 Network activity
3.3 Rootkit functionality
3.4 Command and control (C&C) server
3.5 Disables security software
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 can mitigate the risk of infection by being careful about clicking links found on websites, such as blogs and forums where there is potentially little control or quality checks on the content. Basic checks such as hovering with the mouse pointer over the link will normally show where the link leads to. Users can also check online Web site rating services, such as safeweb.norton.com, to see if the site is deemed safe to visit.

When performing searches in search engines, treat any results returned with caution and double-check them before following the links. If pop-up advertisements are displayed, do not click on them or follow any links within them.

Users should be wary of any sites or services offering free downloads of copyrighted content, such as music, videos, or cracked software. These are often booby-trapped with malicious software and are a known method by way of which this threat can spread. Promiscuous file-sharing may also increase the risk of compromise.

1.2 Patch operating system and software
Users are advised to ensure that their operating systems and any installed software are fully patched, and antivirus and firewall software are up to date and operational. Users are recommended to turn on automatic updates if available so that their computers can receive the latest patches and updates when they are made available.

This threat is known to be spread by through the Blackhole Exploit Toolkit and the Bleeding Life Toolkit, which exploits certain vulnerabilities. Installation of the following patches will reduce the risk to your computer:

1.3 Address blocking
Block access to the following addresses using a firewall, router or add entries to the local hosts files to redirect the following addresses to

As this threat is a Trojan, by definition it doesn't actively spread by itself. Therefore, it needs to use other methods to arrive on a compromised computer. Most commonly, Zeroaccess is spread through websites that have been compromised and redirect traffic to a malicious website that then in turn distribute it using the Blackhole Exploit Toolkit and the Bleeding Life Toolkit. These toolkits then attempt to exploit various vulnerabilities to penetrate the computer and infect it with Zeroaccess.

It has also been observed updating itself through peer-to-peer networks. This allows the creators to continually improve functionality of the threat as well as potentially add new functionality.


The primary motivation of this threat is to make money through pay per click advertising and bitcoin mining. It does this by downloading additional software that conducts Web searches and clicks on the results or mines bitcoins. It attempts to stay hidden and undetected for as long as possible to maximize revenue generation opportunity. It does this by employing advanced rootkit techniques that hide not only the threat itself, but also any other threats that Zeroaccess may download and install.

Furthermore, it opens a back door and connects to a command and control (C&C) server, which allows the remote attacker access to the compromised computer. The attacker is then able to perform any number of actions on the computer, and the computer may then become part of a wider botnet.

Note: Side effects created by associated threats are not included in this report.

The following side effects may be observed on computers compromised by members of threat family. It should be noted that as this threat uses advanced stealth techniques to hide itself, any system modifications may not be visible on the compromised computer except where specialist tools are used to reveal them.

Files/folders created

Files/folders deleted

Files/folders modified
A driver file that is located alphabetically between %System%\Drivers\classpnp.sys and %System%\Drivers\win32k.sys is overwritten with the Trojan's own code.

Registry subkeys/entries created
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\[FILE NAME OF INFECTED DRIVER]\"ImagePath" = "\*"

Registry subkeys/entries deleted

Registry subkeys/entries modified (final values given)

The threat injects its code into the following process:

The threat may be controlled remotely by a command and control (C&C) server. In particular it may be instructed to download and install various other malware. Symantec has observed Trojan.Downbot, aka “Shady RAT,” and W32.Imaut being downloaded by Zeroaccess.

The threat may perform the following network activities.

Zeroaccess may download and execute potentially malicious files. It also may download updates of itself through peer-to-peer networks. It has been observed downloading files from the following IP address:

The threat has been observed contacting the following IP address:

Note: The IP addresses used by this threat change frequently and there is a possibility the threat contacts a large number of locations.

The Trojan may send requests to the following location:

Other network activity
The threat may also attempt to contact one of the following time servers:
  • ntp2.usno.navy.mil
  • ntp.adc.am
  • chronos.cru.fr
  • wwv.nist.gov
  • clock.isc.org
  • time.windows.com
  • time2.one4vision.de
  • time.cerias.purdue.edu
  • clock.fihn.net
  • ntp.duckcorp.org
  • ntp.ucsd.edu
  • ntp1.arnes.si
  • ntp.crifo.org
  • tock.usask.ca

3.3 Rootkit functionality
searches for a driver that is located alphabetically between %System%\Drivers\classpnp.sys and %System%\Drivers\win32k.sys and overwrites the file with its own code, saving the original driver file in another location. The Trojan intercepts all traffic and if an attempt to read or write the infected driver is intercepted, the rootkit fakes the file content by showing the original clean copy of the driver.

ZeroAccess sets up a new RC4 encrypted hidden volume in the computer's file system, which makes its files inside the encrypted volume totally inaccessible to the operating system. The rootkit also allows the threat to download other potentially malicious files and store them inside the hidden rootkit volume, so that they are also hidden from both the user and security software.

3.4 Command and control (C&C) server
The threat attempts to communicate with a predetermined list of C&C servers. It bypasses less intelligent firewalls and security software and sends an encrypted request to all the servers in the list through TCP port 13620.

3.5 Disables security software
The threat creates a fake process and when a security application attempts to access that process during a system scan, the threat stops the security application, which in turn causes the application to crash. The threat then modifies the security application settings so that it cannot run again unless the settings are restored to their default settings.

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: Jarrad Shearer
Summary| Technical Details| Removal

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