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Netskope Threat Research Labs recently posted an article, detailing the CloudPhishing Fan-out resulting from Decoy PDF documents. We are observing a growing trend in the use of such PDF decoys that use Cloud Storage services to carry out not only phishing attacks but also infect user devices with malware such as Remote Administration Tools (RATs). These latest threats are taking advantage of many companies’ “default allow” policy for Cloud Storage services and also the use of popular PDF readers. In our research, we identified several PDF decoys using popular Cloud Storage services like Google Drive, OneDrive (and OneDrive for Business), Box, and Dropbox for downloading malicious payloads. We also identified the usage of URL redirection services for retrieving malicious payloads hosted in Cloud Storage services.
In one particularly clever example, the attack delivers a too-small image of a document, and offers the user a magnifier to trick him/her into clicking on a document, as shown in Figure 1.
Figure 1: Enticing document delivered when the user clicks the magnifier
On clicking the “zoom in/out” magnifier option in Figure 1, the victim is pointed to a malicious payload hosted in Dropbox. At the time of analysis, the Dropbox link was down and not serving malware.
The above illustrates one of the tactics and we have been observing a number of similar themed PDF decoys as shown in Figure 2.
Figure 2: Messages displayed to trick users into launch PDF decoy documents
In the following sections of this blog we will detail two cases in which the PDF decoys used Cloud Storage services to download malicious payloads. The malicious payloads resident in the Cloud Storage services as well as an attempt to download them are detected by Netskope Threat Protection as Backdoor.Gen.20002647 and Backdoor.Generckd.3640291.
Delivery of PDF Decoys via E-mail and Cloud Storage services
During our analysis, we found PDF decoys being sent as email attachments to users at various enterprises. Some organizations tend to save email attachments in Cloud Storage services to reduce email storage, collaborate with clients and colleagues more efficiently, and send and receive large email attachments with greater speed and ease. If a PDF decoy gets synced to the Cloud Storage service and then others sync it to their devices in order to collaborate, that can create a dangerous fan-out effect. See how the attack chain also causes a fan-out effect for email attachments within shared users in the cloud, and how enterprises can protect themselves using Netskope Threat Protection, in Figure 3.
Figure 3: Protection using Netskope Threat Protection
Saving email attachments and collaborating on them in the cloud without regular supervision results in a secondary propagation vector of the decoy document creating a severe impact to an organization.
PDF decoys using Cloud Storage services for downloading malicious payloads with UUE extension
As part of our research, we found several PDF decoys arriving in email that are capable of downloading additional payload files with the UUE extension. As an example, in one case we observed an email with attachment was sent to a contact at a multinational company as shown in Figure 4.
Figure 4: PDF decoy sent as an email attachment
The email attachment contained a PDF decoy named, Swift__Dec2016.pdf. Upon opening the PDF decoy and clicking anywhere in the document, a malicious RAR compressed archive with the UUE extension, named Swift_Dec2016_pdf.uue, hosted in Dropbox is downloaded to the victim’s machine. The compressed archive, Swift_Dec2016_pdf.uue, contains an executable file called Swift_Dec2016_pdf.exe. This compressed archive file resident in the cloud storage as well as the download activity by a user are detected by Netskope Threat Protection as Backdoor.Gen.20002647. This activity is shown in Figure 5.
Figure 5: Swift__Dec2016.pdf retrieving the malicious payload from Dropbox
UUE file extension
Generally, UUE files are binary files that have been encoded, so they can be transmitted as text. Traditional compression tools like WinRAR and WinZip support the decoding of the UUE file format. Since RAR decompression is also supported by these tools, the malicious RAR compressed archive with a UUE extension detonates successfully when the victim inadvertently executes the file. We suspect that the malware author used the UUE file format with the intention of bypassing network security devices.
PDF decoys use URL redirection services to point to Cloud Storage services that deliver malicious payloads
As we continued our research, we identified several PDF decoys that connected to Cloud Storage services via URL redirection services. An example of a PDF decoy that we observed using a URL redirection service to download the malicious payload hosted in Cloud Storage services is shown in Figure 6.
