Guildma (Astaroth) Malware: A New Wave of Infection via Brazilian Portuguese Emails

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Campaign or TTP Overview

In a recent campaign analyzed by the SANS Institute’s Internet Storm Center, the Guildma malware—also known as Astaroth—has been spreading through phishing emails written in Brazilian Portuguese. The attack primarily targets Brazilian users, focusing on email distribution to deliver its malicious payload effectively. This wave of infection was observed in late 2023 and showcases the continual evolution of regional threat tactics that exploit language and cultural nuances to bypass user defenses.

The attackers behind the Guildma malware leverage specific linguistic and social engineering methods to enhance the credibility of their phishing attempts. By crafting emails that appear legitimate and contextually appropriate to the Brazilian audience, they increase the likelihood of user interaction. The primary targets are believed to include individuals and organizations within Brazil, given the localized language usage and contextual familiarity embedded in the emails.

How It Was Built

The construction of the Guildma campaign involves several key components, carefully designed to deceive victims into executing the malware. The infrastructure setup includes domain registration that mimics legitimate Brazilian services, ensuring the URLs appear credible at first glance. The delivery mechanism is primarily through phishing emails that utilize common Brazilian service themes such as banking, e-commerce, and government communications. Here’s a breakdown of their approach:

Subject: Atualização Importante: Sua Conta Será Suspensa

From: suporte@banco-seguro.com.br

To: [target email]

Body: Prezado cliente, devido a recentes atualizações de segurança, é necessário que você confirme suas informações para evitar a suspensão da sua conta. Clique no link para proceder.

Link: http://banco-seguro.com.br.secure-update.com/verify

The lure content is crafted to imitate official notifications or alerts, often invoking urgency and a call to action such as updating account details or verifying identities. The payload is typically disguised as a legitimate software update or document requiring a download, embedded within the phishing site reached via a deceptive URL.

Why It Worked

This campaign’s effectiveness can be attributed to several precise mechanisms that exploit both technical and psychological vulnerabilities:

Localized Language Use: By using Brazilian Portuguese, the attackers tailor their approach to the linguistic environment of the victims, making the emails appear legitimate and avoiding immediate suspicion often triggered by foreign language errors.
Lure Relevance: The themes chosen—banking updates and government notices—tap into the personal and financial interests of the victims. These topics naturally command attention and prompt action due to the perceived risk or benefit associated with them.
Domain Spoofing: Crafting URLs that mimic well-known Brazilian service domains (

banco-seguro.com.br

) plays on the familiarity users have with these services, reducing scrutiny of the link’s authenticity and increasing the likelihood of link clicks.

Operator Takeaways

For red team operators, this campaign provides several adaptable techniques to enhance phishing simulation effectiveness:

Language Localization: Craft phishing emails in the target audience’s native language to reduce detection and enhance authenticity.
Contextual Lure Themes: Select themes that resonate with the specific concerns or interests of the target audience for higher engagement rates.
URL Crafting: Utilize domain names and structures that closely mimic real-world counterparts to exploit user trust and reduce suspicion.

Good / Better / Best

Good: Basic translation of phishing content into the target audience’s language without considering cultural nuances.
Better: Context-aware language usage with themes relevant to the audience’s current socio-economic concerns.
Best: Integrating all aspects of cultural intelligence, including language, local concerns, and domain mimicry to create a seamless and convincing social engineering scheme.

References

For more details on the Guildma malware campaign, refer to the SANS Internet Storm Center analysis.