Hackers Deliver STRRAT Malware Using Weaponized PDF Files

A versatile Java-based RAT that is capable of keylogging and credential theft from browsers and email clients emerged in 2020 that is dubbed “STRRAT.”

The most recent updated version of STRRAT evolved dramatically, and since its discovery, it has been observed that it now does the following things:-

  • Incorporates “Crimson” Ransomware module.
  • Deploys a multitude of infection chains.

A new technique that involves two string obfuscation methods has been recently identified by the cybersecurity researchers at Cyble Research And Intelligence Labs (CRIL) to distribute STRRAT (version 1.6).

Malware Infection chain

With a spam email posing as an electronic company, the infection chain begins, and here the email includes a PDF invoice attachment sent to the target.

Infection chain (Source – Cyble)

When the attached PDF is opened it displays a download image that prompts the user to click on it, which initiates the download of “Invo-0728403.zip” from the following URL:-

  • hxxps://tatchumbemerchants[.]co.ke/Invo-0728403[.]zip
Malicious PDF attachment (Source – Cyble)

Downloaded Zip holds encrypted STRRAT payload in JavaScript. Upon execution, JS decrypts the payload, placing “lypbtrtr.txt” in the following directory:-

File type check reveals a disguised JAR (zip) file that extracts the “carLambo” folder and META-INF with classes, resources, and MANIFEST.MF which ensures it is “STRRAT malware.

Content of JAR file (Source – Cyble)

The analysis of the latest variant of STRRAT malware shows class name modifications and two string obfuscators (Allatori, ZKM) used, unlike the prior version that used only “Allatori.”

Distribution & Persistance

Since March 2023, the STRRAT malware (version 1.6) is actively distributed through multiple infection chains, and not only that in the wild more than 70 samples were detected.

For persistence, it sets the “Skype” task scheduler entry, and STRRAT 1.6 stores C&C server info in an encrypted Base64-encoded config.txt file with AES encryption, as in previous versions.

Here below we have mentioned the browsers that are targeted:-

  • Chrome
  • Firefox
  • Internet Explorer 

Here below we have mentioned the email clients that are targeted:-

  • Outlook
  • Thunderbird
  • Foxmail

Recommendations

Here below we have mentioned all the recommendations that are offered by the security analysts:-

  • Always use strong email filter solutions.
  • Make sure to verify the links and attachments before opening them.
  • Always use robust endpoint security solutions. 
  • Keep security tools updated with the latest available patches and updates.
  • Implement URL filtering to block malicious sites.
  • Conduct regular employee cybersecurity training.

IoCs

IoCs (Source – Cyble)

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