Home > Articles > Home & Office Computing > Mac OS X

Alternate Data Streams: Threat or Menace?

By now, you're probably aware of how easy it is to accidentally leave metadata in your Microsoft Word documents, offending or amusing recipients who can easily uncover your mistakes. Did you also know that bad guys might be helping you to even more embarrassment (or danger), by using the alternate data streams (ADS) capability of Windows NTFS to hook warez, pornography, or even hacker's tools to your innocent files?
Like this article? We recommend

Alternate data streams (ADS) are an example of how a good idea is compromised by lousy execution. Alternate data streams provide an easy way to invisibly attach metadata to Windows files.

Invisibly. And that's the problem. ADS attachments don't show up in file directories, they don't change the reported size of the files, and in fact they're practically undetectable with the standard Windows tools. As a result, alternate data streams have become the tool of choice for computer criminals of all sorts, from crooks stealing your data to bad guys who want to turn your system into their zombie—the ones who use your system to host secret chat rooms and bulletin boards full of illegal pornography.

While you can't disable alternate data streams on a Windows system, some tools will let you spot questionable ADS attachments and control them. However, to protect yourself against ADS, you have to understand the facts about this remarkably little-known threat.

Why Alternate Data Streams?

Alternate data streams were added to the NTFS filesystem primarily for Macintosh compatibility. The Macintosh Hierarchical File System uses attached metadata (called the resource fork—Microsoft doesn't have a monopoly on goofy feature names) to tell the system how to handle the data (called the data fork). In effect, it's a more powerful, flexible version of the dot-3 extensions (.doc, .txt, .bmp, etc.) on Windows filenames. Having an equivalent to the resource fork in NTFS made it a lot easier to port software between the operating systems.

Like the resource fork in the Mac, ADS can include a lot more information about a file than just the file type. This is why a number of Windows programs use alternate data streams. A good example of ADS use is provided by Microsoft Word. Every Word document can have annotations attached describing the document, and this information is stored in an ADS attached to the document.

However, in implementing ADS, Microsoft made a couple of decisions that were, to put it mildly, questionable. The first decision was to allow any kind of file of nearly any size to be attached as an alternate data stream. This includes executables, which can run quite cheerfully from the ADS. A file of even a few bytes can have an ADS totaling hundreds of megabytes in length. That's a lot of warez, pornography, or hacker's tools.

The second bad decision was to render alternate data streams invisible to standard Windows tools such as Internet Explorer and the dir command. When you attach an ADS to a file, the reported file size remains the same. The only thing that changes is the creation date on the file. chkdsk will show that more space has been consumed on the disk, but it won't show you where the space went.

While alternate data streams are hard to detect, they're easy to create. Using a few simple commands, anyone can attach an ADS to a file. Their ease of creation combined with their ability to contain just about anything means that alternate data streams are an ideal tool for hiding something in Windows. Not surprisingly, it didn't take all sorts of bad guys long to figure that out. Today ADS is one of the most common methods of hiding stuff in compromised systems.

What kinds of things use ADS? You'd be astonished—and appalled. In general, any piece of software that wants to hide itself is likely to use ADS. Trojans and rootkits are particularly prolific users because alternate data streams provide an easy way to hide things.

A system breaker needs tools to control and manipulate a compromised computer. ADS provides an ideal way to hide the tools that can be used to do further damage to your system—on your system. By keeping the toolkit on the compromised system, the system breaker greatly reduces the bandwidth needed for repeat visits. A couple of simple scripts totaling a few kilobytes replace downloading megabytes of software every time the bad guy visits.

Of course, a smart computer criminal doesn't keep anything incriminating on his computer. He hides it on someone else's computer, often using ADS. It's not uncommon to find all kinds of files stolen from other computers parked on some innocent's system and hidden with ADS.

ADS is also used to hide the code to run illicit chat rooms, FTP servers, and other covert communications channels on compromised systems.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020