The Water Saci threat actor has evolved its malware delivery tactics by deploying a banking Trojan through a highly sophisticated, multi-layered infection chain. The infection begins with the distribution of HTML Application (HTA) files and PDF documents that act as initial infection vectors. These files propagate a worm via WhatsApp, exploiting the platform's messaging capabilities to spread rapidly among contacts. The latest variant replaces the previously used PowerShell scripts with a Python-based malware, indicating a shift towards more flexible and potentially harder-to-detect payloads. Additionally, the campaign incorporates RelayNFC techniques, which enable NFC relay fraud by intercepting and relaying near-field communication signals, potentially bypassing security controls on contactless payment systems. This combination of social engineering, multi-stage infection, and NFC relay fraud allows the attackers to harvest banking credentials and perform unauthorized transactions. While the campaign is currently focused on Brazilian users, the underlying techniques could be adapted to other regions with similar mobile payment and messaging ecosystems. The absence of known exploits in the wild suggests the attack relies heavily on user interaction and social engineering rather than software vulnerabilities. The use of WhatsApp as a propagation vector leverages its widespread adoption, making the worm highly effective in spreading laterally through social contacts. The Trojan’s focus on banking credentials makes it a significant threat to financial institutions and their customers, potentially leading to financial losses and reputational damage.

For European organizations, this threat poses a significant risk primarily through the potential targeting of customers or employees who have connections to Brazil or use WhatsApp extensively. Financial institutions in Europe could face indirect impacts if their customers are compromised or if attackers adapt these tactics to European banking environments. The NFC relay fraud component is particularly concerning for European countries with widespread adoption of contactless payment technologies, as it could enable unauthorized transactions and fraud. The worm’s propagation via WhatsApp also raises concerns about rapid lateral spread within organizations and personal networks, increasing the risk of credential theft and subsequent fraud. The multi-layered infection chain complicates detection and response, potentially leading to prolonged dwell times and increased damage. Furthermore, the use of Python-based malware may evade traditional signature-based detection tools, requiring more advanced behavioral analysis. Overall, the threat could disrupt banking operations, erode customer trust, and result in financial losses if not mitigated effectively.

European organizations should implement several targeted measures beyond generic advice: 1) Deploy advanced email and messaging filtering solutions capable of detecting and blocking HTA files, malicious PDFs, and suspicious links, especially those propagated via WhatsApp. 2) Enhance endpoint detection and response (EDR) capabilities to identify and quarantine Python-based malware and unusual script execution. 3) Conduct focused user awareness training emphasizing the risks of interacting with unsolicited HTA and PDF attachments and messages received through WhatsApp. 4) Monitor NFC transaction logs and implement anomaly detection to identify potential relay fraud activities, including unusual transaction patterns or device behaviors. 5) Collaborate with mobile network operators and payment providers to detect and mitigate NFC relay attempts. 6) Enforce multi-factor authentication (MFA) for banking and critical systems to reduce the impact of credential theft. 7) Establish incident response playbooks specifically addressing multi-stage infections and social engineering attacks involving messaging platforms. 8) Regularly update and patch all systems, including mobile devices, to reduce the attack surface. 9) Engage in threat intelligence sharing with European CERTs and financial sector ISACs to stay informed about evolving tactics related to Water Saci and similar threat actors.