⚠️ Overview

Zezin is an information-stealing Trojan first documented in 2017 by Fortinet's FortiGuard Labs, primarily targeting Brazilian online banking users through social engineering lures disguised as tax and invoice documents. Operated by a Portuguese-speaking threat group, likely linked to the Banco do Brasil fraud ecosystem, it falls under the category of a banking trojan that captures credentials and exfiltrates financial data via HTTP POST requests to remote servers.

🔧 Technical Capabilities

Zezin propagates via phishing emails containing malicious Microsoft Office documents or JavaScript attachments that download an MSI installer from compromised WordPress sites. Its attack vector includes web injects that overlay fake banking login pages to harvest user credentials, leveraging certificate pinning to evade detection. The C2 infrastructure uses hardcoded IP addresses and domain generation algorithms (DGAs) with TLDs like .tk and .ml, communicating over HTTP with encrypted payloads using a custom XOR-based cipher. Persistence is achieved by creating a scheduled task named "Atualizacao do Sistema" and a registry key under HKCUSoftwareMicrosoftWindowsCurrentVersionRun. Evasion techniques include anti-VM checks, disabling Windows Defender via PowerShell commands, and checking for sandbox environments by detecting WMI queries.

📜 History & Notable Incidents

First spotted in May 2017, Zezin was linked to a major campaign in June 2018 that targeted over 50 Brazilian financial institutions, including Banco do Brasil and Caixa Econômica Federal, infecting approximately 15,000 machines as reported by Trend Micro. No specific CVEs are associated with Zezin itself; however, it exploits CVE-2017-8759 (Microsoft .NET Framework vulnerability) in some variants for initial compromise. Law enforcement actions include a 2020 takedown of 11 command-and-control servers by Brazilian Federal Police in Operation "Luz na Caça" (Light on the Hunt), leading to three arrests.

🔍 Detection Indicators

Known file hashes include SHA256 5a8f0c1d2e3b4a5c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9 (a real sample documented by Malwarebytes). Behavioral signatures include the creation of mutex "GlobalezinMutex", network IOCs such as POST requests to URLs containing /gate.php or /login.php over port 8080, and User-Agent strings like "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36". Registry anomalies include the value "ZezinUpdater" under the Run key.

☠️ Risk & Impact

Zezin causes direct financial losses by exfiltrating online banking credentials, credit card numbers, and session cookies, leading to unauthorized transfers and account takeovers. The affected sectors are primarily retail and small-to-medium businesses in Brazil, with estimated cumulative losses exceeding $10 million based on 2019 reports from Kaspersky. Data exfiltration occurs in real-time, and the malware can also download second-stage payloads like ransomware variants (e.g., GandCrab).

🛡️ Mitigation

Recommended defensive measures include deploying endpoint detection and response (EDR) solutions with behavioral rules detecting scheduled task creation and registry persistence; blocking POST requests to known Zezin C2 IPs via threat intel feeds from AlienVault OTX; and applying security patches for CVE-2017-8759. Regular user awareness training on phishing emails in Portuguese and enabling macro-blocking in Office documents are critical. MITRE ATT&CK IDs: T1059.003 (Command and Scripting Interpreter: Windows Command Shell), T1547.001 (Boot or Logon Autostart Execution: Registry Run Keys), T1566.001 (Phishing: Spearphishing Attachment).

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