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Netskope Threat Research Labs detected several targeted themed attacks across 42 customer instances mostly in the banking and finance sector. The threat actors involved in these attacks used the App Engine Google Cloud computing platform (GCP) to deliver malware via PDF decoys. After further research, we confirmed evidence of these attacks targeting governments and financial firms worldwide. Several decoys were likely related to an infamous threat actor group named ‘Cobalt Strike’.
The attacks were carried out by abusing the GCP URL redirection in PDF decoys and redirecting to the malicious URL hosting the malicious payload. This targeted attack is more convincing than the traditional attacks because the URL hosting the malware points the host URL to Google App Engine, thus making the victim believe the file is delivered from a trusted source like Google.
This post describes our discovery and analysis of the Google App Engine URL Redirection abuse, the threat actor responsible, and the malware abusing this feature. We conclude with some recommendations to help protect and remediate such threats.
Netskope Detection
Netskope Advanced Threat Protection detects the targeted decoy we identified as PDF_Phish.Gen.
Disclosure
Netskope reported this abuse to Google on 10 January 2019. The open redirector exists by design.
Discovery
Early this year, Netskope’s telemetry identified common detections across 42 customers in the banking and finance sector. All these were eml files that carried an .eml extension and had the same detection name, triggering alerts in our Outbreak Detection Systems. After investigation, we confirmed that the detections were triggered in the attachments of the eml files.
Leveraging our Netskope Discovery and Netskope Active Introspection Alerts platforms, we discovered these attacks were abusing Google App Engine on the Google Cloud Platform (GCP) as a bait to deliver malware.
PDF Decoys – Delivery
The PDF decoys traditionally arrive as email attachments to victims. The emails are crafted to contain legitimate content and deliver the malware from allow listed sources. Often, such attachments are saved to cloud storage services, like Google Drive. Sharing these documents with other users can result in the occurrence of a secondary propagation vector like the CloudPhishing Fan-out Effect. In this case, the email file containing the decoy document was detected by Advanced Threat Protection and the potential fan-out was prevented.
GCP App Engine URL Redirection – Decoys
This targeted attack is more convincing than the traditional attacks because the decoy deceives the victim with a GoogleApp Engine URL which is abused to redirect the victim to the malware. As the payload seems to be originating from a trusted source, the chance of falling victim to such attacks is very likely.
The themed PDF decoys we observed using GCP App Engine URL Redirection is shown in Figure 1.
Most of the PDF’s we observed were created using Adobe Acrobat 18.0. They contained the malicious URL in a compressed form in the PDF stream using Flat Decode (Filter/FlateDecode). Similarly, all the decoys used HTTPS URLs for delivering the payload.
GCP App Engine URL Redirection – Overview
The URL redirection case with Google App Engine falls under the category of Unvalidated Redirects and Forwards as per the Open Web Application Security Project (OWASP).
Some of the URLs we observed from the PDF decoys using this technique are shown below
- https://appengine.google[.]com/_ah/logout?continue=https%3A%2F%2Ftransef[.]biz%2FDoc102018.doc
- https://appengine.google[.]com/_ah/logout?continue=https%3A%2F%2Fswptransaction-scan2034.s3.ca-central-1.amazonaws[.]com%2FDoc102018.doc
For a better understanding and illustration, we have taken the below URL as an example
- https://appengine.google[.]com/_ah/logout?continue=https%3A%2F%2Ftransef[.]biz%2FDoc102018.doc
The packet capture illustrating this activity is shown in Figure 2.
Figure 2 illustrates that once the URL is accessed, the user is logged out from appengine.google.com and a response status code ‘302’ is generated for URL redirection. As this action gets executed, the user is inturn redirected to google.com/url using the query “?continue=”. Using this redirection logic, the destination landing page is reached, and in this case, Doc102018.doc is downloaded to the victim’s machine.
Using the same logic in the query, we tried the same approach with GCP App Engine URL as follows:
- https://accounts.google.com/Logout?continue=https://appengine.google.com/_ah/logout?continue=https%3A%2F%2Ftransef[.]biz%2FDoc102018.doc
- https://google.com/accounts/Logout?continue=https://appengine.google.com/_ah/logout?continue=https%3A%2F%2Ftransef[.]biz%2FDoc102018.doc
In all these cases, the GCP App Engine application successfully validated the redirection and delivered the payload to the victims machine.
Since the appended URL is an unvalidated redirect, the threat actors abused this feature by redirecting a victim to a malicious appended URL hosting the malicious payload.
PDF Decoys – Default Allow policy
Generally, PDF readers prompt a security warning to the user when the document connects to a website. Once “remember this action for this site” is checked for a domain, this feature allows any URL within the domain without any prompt.
The activity illustrating this action is shown in Figure 3.
By taking advantage of the “default allow” action in popular PDF readers, the attacker can easily deploy multiple attacks without getting the security warning after the first alert. It is also possible that appengine.google.com is allow listed by the administrators for legitimate reasons. It also only warns the user that it is trying to connect to appengine.google.com, which looks benign at face value.
Malware using GCP URL Redirection
All the PDFs involved in this attack downloaded Microsoft Word documents with obfuscated macro code or PDF documents as the second stage payload.
The PDF decoy detected in our customer instances downloaded a word document named “Doc102018.doc” containing obfuscated macro code. This document was downloaded from the URL https://transef[.]biz.
On execution, the victim is presented with a message to enable editing and content mode to view the document as shown in Figure 4.
