Windows uses a simple technique to keep track of which binary files were downloaded from the Internet (or a network share).
Each downloaded file is is tagged with a hidden NTFS Alternate Data Stream file named Zone.Identifier. You can check for the presence of this “Mark of the Web” (MotW) using dir /r or programmatically, and you can view the contents of the MotW stream using Notepad:
Within the file, the ZoneTransfer element contains a ZoneId element with the ordinal value of the URLMon Zone from which the file came. The value 3 indicates that the file is from the Internet Zone.
Aside: One common question is “Why does the file contain a Zone Id rather than the original URL? There’s a lot of cool things that we could do if a URL was preserved!” The answer is mostly related to privacy—storing a URL within a hidden data stream is a foot gun that would likely lead to accidental disclosure of private URLs. This problem isn’t just theoretical—the Alternate Data Stream is only one mechanism used for MotW. Another mechanism involves writing a <!–saved from url=> comment to HTML markup; that form does include a raw URL. A few years ago, attackers noticed that they could use Google to search for files containing a MOTW of the form <!—saved from url(0042)ftp://username:firstname.lastname@example.org –> and collect credentials. Oops.
Update: Microsoft apparently either forgot or decided the tradeoff was worth it. Windows 10 includes the referrer URL, source URL and other information.
Browsers and other internet clients (e.g. email and chat programs) can participate in the MOTW-marking system by using the IAttachmentExecute interface’s methods or by writing the Alternate Data Stream directly. Update: Chrome uses IAttachmentExecute and thus includes the URL information on Windows 10. Firefox writes the Alternate Data Stream directly and thus does not.
Handling Marked Files
Windows and some applications treat files with a Mark-of-the-Web differently than those without. For instance, examining a downloaded executable file’s properties shows the following notice:
More importantly, attempting to run the executable using Windows Explorer or ShellExecute() will first trigger evaluation using SmartScreen Application Reputation (Win8+) and any installed anti-virus scanners. The file’s digital signature will be checked, and execution will be confirmed with the user, either using the older Attachment Execution Services prompt, or the newer UAC elevation prompt:
Microsoft Office documents bearing a MotW open in Protected View, a security sandbox that attempts to block many forms of malicious content.
Trivia: Some applications inherit protections against files bearing a MotW, but don’t have any user-interface that explains what is going on. For instance, if you download a CHM with a MotW, its HTML content will not render until you unblock it using the “Always ask before opening this file” or the “Unblock” button:
What Could Go Wrong?
With such a simple scheme, what could go wrong? Unfortunately, quite a lot.
Internet Clients Must Participate
The first hurdle is that Internet clients must explicitly mark their downloads using the Mark-of-the-Web, either by calling IAttachmentExecute or by writing the Alternate Data Stream directly. Most popular clients will do so, but support is neither universal nor comprehensive.
For instance, for a few years, Firefox failed to mark downloads if the user used the Open command instead of Save.
In other cases, some browser plugins may allow attackers to save files to disk and bypass MotW tagging.
Microsoft Outlook (tested v2010) and Microsoft Windows Live Mail Desktop (tested v2012 16.4.3563.0918) both tag message attachments with a MotW you double-click on an attachment or right-click and choose Save As. Unfortunately, however, both clients fail to tag attachments if the user uses drag-and-drop to copy the attachment to somewhere in their filesystem. This oversight is likely to be seen in many different clients, owing to the complexity in determining the drop destination.
Target File System Must Be NTFS
The Zone.Identifier stream can only be saved in an NTFS stream. These streams are not available on FAT32-formatted devices (e.g. some USB Flash drives), CD/DVDs, or the new ReFS file system introduced with Windows Server 2012.
If you copy a file tagged with a MotW to a non-NTFS filesystem (or try to save it to such a file system to start with), the Mark of the Web is omitted and the protection is lost.
Originating Location Must Be Internet or Restricted Zone
The IAttachmentExecute:Save API will not write the MotW unless the URL provided in the SetSource method is in the Internet or Restricted Sites zone.
On some systems, code may have added domains to the user’s Trusted Sites zone without their knowledge.
On other systems, a proxy configuration script may cause Internet sites to be treated as belonging to the Intranet zone.
Origin Laundering via Archives
One simple trick that attackers use to try to circumvent MotW protections is to enclose their data within an archive like a .ZIP, .7z, or .RAR file. Attackers may go further and add a password to block virus scanners; the password is provided to the victim in the attacking webpage or email.
In order to remain secure, archive extractors must correctly propagate the MotW from the archive file itself to each file extracted from the archive.
Despite being one of the worst ZIP clients available, Windows Explorer gets this right:
In contrast, 7-zip does not reliably get this right. Malware within a 7-zip archive can be extracted without propagation of the MotW. 7-zip v15.14 will add a MotW if you double-click an exe within an archive, but not if you extract it first. The older 7-zip v9.2 does not tag with MotW either way.
A quick look at popular archive extractors shows:
- Windows Explorer – Not vulnerable
- WinRar 5.31 – Not vulnerable
- WinZip 20.0.11649 – Not vulnerable
- 7-Zip 15.14 – Vulnerable (bug)
- IZArc 4.2 – Vulnerable (Developer says: “Will be fixed in next version”)
Tested another? Let me know your findings in the comments.
SmartScreen & the User May Unmark
Finally, users may unmark files using the Unblock button on the file’s Properties dialog, or by unticking the “Always ask before opening this file” checkbox. Similarly, on systems with Microsoft SmartScreen, SmartScreen itself may unmark the file (actually, it replaces the ZoneId with an (undocumented) value AppZoneId=4).
Mark-of-the-Web is valuable, but fragile.