ShortJSRAT leverages cloud with scriptlets

Netskope Threat Research Labs has identified a new malware named “ShortJSRAT” which uses a Windows script component scriptlet file with a .sct extension. The scripts we observed used cloud apps for delivering the next stage payloads. These payloads are executed using the “Squiblydoo” technique which use native windows applications to bypass application allow listing solutions like Windows Applocker. This solution allows only approved applications to load and execute. Additionally, threat actors behind this malware used various cloud applications to take advantage of a typical enterprise loopholes of bypassing security inspection for cloud application traffic. Netskope Threat Protection detects these scriptlets as Squiblydoo.ShortJSRAT.Gen1.

As part of our research efforts, we discovered multiple variants of ShortJSRAT malware using the Squiblydoo technique. This powerful technique used by threat actors uses various components such as trusted Microsoft binary, proxy, TLS encryption and webpage redirection allowing an attacker with normal privileges to download and execute a scriptlet from a remote host.Since, ShortJSRAT also downloads and executes scriptlets hosted on Cloud Apps, this makes traditional technologies which generally rely on static detections harder to detect and remediate the attack. One of the samples we discovered used Dropbox via Google URL shortener, while another sample used Google Storage APIs to download the scriptlet files. Another variant we discovered used Github as part of the attack kill chain along the lines of another interesting malware that we discovered recently which used Github for malware payload delivery. Based on our observation of ShortJSRAT using many cloud services for additional resiliency, enterprises need a CASB to efficiently analyze the unsanctioned stream context.

A typical delivery of ShortJSRAT and some of the attack kill chain phases are shown in Figure 1. A victim typically receives an email with a Google URL Shortener link and clicking on the link ultimately lead to the delivery of the ShortJSRAT malware.

Figure 1: Depiction of the ShortJSRAT attack kill chain

We identified ShortJSRAT as part of our monitoring efforts to identify new developments and evolution of CloudSquirrel and CloudFanta malware campaigns that we uncovered in 2016. Our investigation revealed similarities between ShortJSRAT scriptlet and some of the malware from CloudFanta campaign. The comparison of the code between CloudFanta malware and ShortJSRAT scriptlet is shown in Figure 2.

Figure 2: Comparison of CloudFanta and ShortJSRAT scriptlet

At this time we do not have enough evidence to claim that ShortJSRAT is the next version of CloudFanta.

As per our observation, ShortJSRAT is likely written or exclusively used by Brazilian threat actors. These threat actors use cloud services to download and deliver the malware and its payloads via a portable executable file ( like DLL and EXE ) with a “.png” extension. There were also some variants that downloaded 32-bit or 64-bit executables with a check of file path “c:\\Program Files (x86)” to determine if the victim is using a 32-bit or 64-bit operating system.

Netskope Threat Detection and Protection

Netskope Threat Protection detects these scriptlets as Squiblydoo.ShortJSRAT.Gen1. Additionally, Netskope customers can enable cloud app instance-level policies to block downloads from unsanctioned cloud storage applications while allowing full access to enterprise sanctioned instances. This can aid Netskope customers in preventing this attack with app instance-level granularity. Applying such policies enables the administrator to permit corporate instances of cloud storage and collaboration apps such as Dropbox and Github, while preventing access to other hostile instances. An example of a policy preventing download of malicious files from cloud applications is shown in Figure 3.

Figure 3: App instance-level policies to block downloads from unsanctioned cloud apps

Conclusion

Threat actors have an innate interest in leveraging the cloud for deploying evasive and innovative tactics. The delivery of scriptlets using cloud apps like Github and Dropbox and armouring them with evasive techniques like Squiblydoo is one such intuitive indicant.We clearly foresee an increase in sophisticated attacks using the cloud, as the threat actors are fastly adapting the cloud as a new attack vector. This is a serious concern and augmented challenge for enterprises. Netskope Threat Protection is the only solution that performs in-depth inspection at an instance level which aids in enabling cloud app instance-level policies. Such features provide enterprises with a multi-layered protection to detect and stay protected from threats and threat actors using the cloud. Netskope has worked with Dropbox and GitHub security team to take taken down the attack elements of ShortJSRAT.

General Recommendations

Netskope recommends the following to combat cloud malware and threats:

• Detect and remediate cloud threats using a threat-aware CASB solution like Netskope and enforce policy on usage of unsanctioned services as well as unsanctioned instances of sanctioned cloud services
• Sample policies to enforce:
  • Scan all uploads from unmanaged devices to sanctioned cloud applications for malware
  • Scan all uploads from remote devices to sanctioned cloud applications for malware
  • Scan all downloads from unsanctioned cloud applications for malware
  • Scan all downloads from unsanctioned instances of sanctioned cloud applications for malware
  • Enforce quarantine/block actions on malware detection to reduce user impact
  • Block unsanctioned instances of sanctioned/well known cloud apps, to prevent attackers from exploiting user trust in cloud. While this seems a little restrictive, it significantly reduces the risk of malware infiltration attempts via cloud
• Enforce DLP policies to control files and data en route to or from your corporate environment
• Regularly back up and turn on versioning for critical content in cloud services
• Enable the “View known file extensions” option on Windows machines
• Warn users to avoid executing unsigned macros and macros from an untrusted source, unless they are very sure that they are benign
• Whenever you receive a hyperlink, hover the mouse over it to ensure it’s legitimate
• Administrators can also consider creating a security policy to block PE files, SCT files and non PNG files with content type ”image/png”
• Administrators can also consider to Improve credential protection for Microsoft Windows
• Warn users to avoid executing any file unless they are very sure that they are benign
• Warn users against opening untrusted attachments, regardless of their extensions or filenames
• Keep systems and antivirus updated with the latest releases and patches

Technical Analysis

Analysis of the attack kill chain

As mentioned in Figure 1, the Google URL shortener link redirects to Dropbox cloud storage service to download a first stage downloader scriptlet file, 1.sct as shown in Figure 4.