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Netskope Threat Research Labs has observed phishing attacks using decoy PDF files, URL redirection, and Cloud Storage services to infect users and propagate malware. Because many organizations have default “allow” security policies for popular Cloud Storage services and PDF readers to let users take advantage of these useful services, these attacks pass through the corporate network to end users’ machines undetected. Moreover, as users collaborate and share through cloud services, these malicious files posing as PDFs “fan out” to shared users, creating a secondary propagation vector. We are calling this the “CloudPhishing Fan-out Effect.”
In this blog, we will detail the insidious nature of CloudPhishing and the secondary fan-out using two recently detected cases. We will also illustrate how an attack – even if unsuccessful – may leave the target vulnerable to future attacks. Additionally, we will outline the Netskope protection stance, and general best practices to handle this attack.
CloudPhishing Fan-out
The CloudPhishing fan-out effect occurs when a victim inadvertently shares the phishing document with colleagues, whether internal or external, via a cloud service. This is particularly insidious in the cloud, as shared users lose the context of the document’s external origin and may trust the internally shared document as if it were created internally. The visual depiction of the attack chain showing the CloudPhishing fan-out is shown in Figure 1.
Figure 1: CloudPhishing fanout effect within shared users in the cloudOne example of this threat is an invoice decoy document named “Invoice_2884.pdf” (MD5 – 987a95d47ea99fe32e4ce095c591383f), which has been shared in Microsoft OneDrive for Business. Other than having the file shared in OneDrive, the SaaS application is unrelated to the attack. This threat, seen in one of our customer environments, is detected by Netskope Active Threat Protection as Backdoor.Phishing.FW.
The decoy PDF is usually delivered as an email attachment named,“invoice” in an attempt to lure the victim into executing the file. When the user opens the PDFdocument and inadvertently clicks the hyperlink in the PDF, the decoy PDF retrieves a phished page referenced by the TinyURL service, as shown in Figure 2.
Figure 2: Visual depiction of the working of the PDF decoy, Invoice_2884.pdf
Analysis of the PDF decoy, Invoice_2884.pdf
On execution of the PDF decoy, “Invoice_2884.pdf,” the victim is presented with a secured PDF online document message, luring him or her to click on the “View On Adobe” hyperlink to view the document, as shown in Figure 3.
Figure 3: Message displayed on opening Invoice_2884.pdf
Upon clicking the “View on Adobe” hyperlink, the victim is displayed with a security warning message from Adobe that the document is trying to connect to the TinyURL redirection service link, as shown in Figure 4.
Figure 4: Invoice_2884.pdf connecting to TinyURL redirection service link
If this action is allowed without un-checking the “Remember this action…,” all future connections to TinyURL.com will be allowed without any security warning prompts from the victim’s machine, as shown in Figure 5. This, in effect, weakens the security posture of the endpoint against future attacks.
Figure 5: Connection to TinyURL.com set to “Always Allow”
Allowing this action paves a pathway for the attacker to carry out future actions by launching decoys from the allowed domains list.
The decoy PDF connects to the TinyURL link, http://TinyURL[.]com/he9n53x, as shown in Figure 6.
Figure 6: Hyperlink in the PDF decoy, Invoice_2884.pdf
The TinyURL link redirects to the link, http://www.pdf.anothersideofpeace[.]org/PO.htm. The attacker used the TinyURL link as an evasive tactic to hide the original link. At the time of analysis, the web page was down and not serving any content. This might be because the web page was removed or renamed. The web page was also blocked by the Chrome browser, as shown in Figure 7.
Figure 7: Blocked web page by Chrome browser
Similarly-themed PDF decoy strains
We identified similar strains of decoy PDF documents that retrieve phished pages hosted in several URL redirection services and Cloud Storage services. The visual depiction of protection via Netskope Active Threat Protection for the PDF decoy using Cloud Storage services for retrieving the phished pages and the CloudPhishing fan-out effect is shown in Figure 8.