The STR RAT is a remote access trojan (RAT) written in Java, first detected in 2020. Like other RATs, it allows threat actors full control of an infected machine. STR RAT is capable of keylogging, credential theft, and deploying additional malicious payloads. 

The malware is updated annually, aligning with its renewed use by threat actors. Cofense's analysis from January 2023 to April 2024 reveals that 60% of STR RAT samples are delivered directly via email rather than embedded links.

STR RAT resembles a seasonal flu, with yearly updates making it more prominent for short periods. Initially discovered on an antivirus forum in 2020, version 1.2 already featured keylogging, password theft, and backdoor access, along with a fake “.crimson” ransomware module that only renamed files. In 2021, Microsoft Threat Intelligence highlighted STR RAT in phishing campaigns. By 2022, it spoofed the Maersk shipping brand and employed a polyglot file technique, allowing execution as an MSI or Java file. In 2023, version 1.6 used Zelix KlassMaster and Allatori for code obfuscation. In 2024, STR RAT was uploaded to legitimate services like GitHub and AWS, making it harder to detect.

STR RAT steals passwords from Chrome, Firefox, Internet Explorer, and email clients like Outlook, Thunderbird, and Foxmail. Key commands include o-keylogger for logging keystrokes, down-n-exec for file execution, remote-screen for commandeering the computer, and power-shell for PowerShell access.

Though not as prevalent as other RATs like Remcos, STR RAT showed sustained activity from March to August 2023, likely due to the new version and polyglot file technique. In March 2024, significant activity was noted again, attributed to the use of legitimate services like GitHub and AWS for hosting and delivering the malware. STR RAT is typically delivered via email as an archive containing a .jar file, requiring a Java Runtime Environment (JRE) to execute. These archives may also contain necessary JRE binaries or download them from Maven and GitHub repositories.

STR RAT's second most common delivery mechanism is loaders, which reach out to a payload location to download and run the malware. Jar Downloaders, CVE-2017-11882 exploits in Microsoft Office, and Windows Registry File downloaders are commonly used loaders. Additionally, embedded URLs in emails or attached PDFs often lead to the malware hosted on legitimate services like AWS, GitHub, and Discord’s CDN.

Unlike loaders, droppers contain the malware to be deployed. STR RAT's most common dropper is the JavaScript Dropper (JS Dropper), a .js file that executes natively on Windows. JS Droppers are usually attached to emails and contain both the dropper and STR RAT.

Upon execution, STR RAT places files, creates persistence, and installs dependencies. It uses geolocator services to geo-fingerprint infected computers and sends system information to its command-and-control (C2) server. The malware also uses legitimate Java libraries for keylogging and database connectivity.

Different versions of STR RAT leave various indicators of compromise (IOCs). After execution, STR RAT copies itself to multiple locations, creates a \lib\ folder with legitimate files, and generates a XXXXlock.file in the user's local home profile. The configuration can be observed through memory analysis, revealing the C2 server, port, and domain.

STR RAT can create persistence through Registry Run Keys, Startup Folder entries, or Scheduled Tasks, ensuring the malware runs every time the user logs in. Endpoint detection and response software can monitor specific locations for signs of STR RAT persistence.

STR RAT communicates with C2 servers using subdomains of free dynamic DNS services and legitimate services like GitHub and Maven. HTTP is used for C2 communications, though the port is not the standard tcp/80.

STR RAT reaches out to legitimate services for hosting tools and malware. Indicators of suspicious activity include access to GitHub and Maven repositories in conjunction with other malicious behaviors.

By understanding STR RAT's history, capabilities, and delivery mechanisms, cybersecurity professionals can better detect and defend against this persistent threat.