Rewterz Threat Alert – GuLoader: a Shellcode-based Downloader

Severity

Medium

Analysis Summary

Guloader is currently being distributed via spam email campaigns with archived attachments that contain the malware. The majority of malware downloaded by GuLoader is commodity malware, with AgentTesla, FormBook and NanoCore being the most predominant. This downloader typically stores its encrypted payloads on Google Drive. CrowdStrike has observed that GuLoader downloads its payloads from Microsoft OneDrive and also from compromised or attacker-controlled websites. By utilizing legitimate file-sharing websites, GuLoader can evade network-based detection, as these services are not generally filtered or inspected in corporate environments. In addition, the downloaded payloads are encrypted with a hard-coded XOR key embedded in the malware, making it difficult for file-sharing service providers to identify the payload as malicious.

Impact

• Malware installation
• Detection evasion
• Information theft

Indicators of Compromise

Email Subject

RE[:] PAYMENT TO BANK DETAILS (CONFIRM BANK DETAILS)

MD5

• 34b71a0bf9335a274086183ef09a769e
• 7db83b17d0e17975960caf1f09559861
• 1ae080b109bc5ab7cfd42e5d02b12437

SHA-256

• 38e6cef6c556cb8ce5254876fd43caf59bbb8239a1ea679891a4d423aafb08dc
• c61f1d14582a38474f56426975cc4a2b2fa9ff172c915af9781c9d5682cb629e
• bfa5dba46db1253587058b0392c04c8403846fa55d7dcf1044e94e6a654d4715

SHA1

• 00386297ef58e8fabba4d7f0d875a91c6ed846cb
• 40df6148b8f14b63feba62aba40da180e9c50f47
• 0e48b88e2a79cc437bfd4934064e2779da9940b8

Remediation

• Block the threat indicators at their respective controls. 
• Beware of invoice themed emails coming from untrusted sources.