The malicious Sicoob.Sdk NuGet package is another reminder that software supply-chain attacks are now targeting business integrations, not just generic developer environments. According to the report, the package impersonated a C# SDK for Sicoob, one of Brazil’s largest cooperative financial systems, and versions 2.0.0 through 2.0.4 were designed to steal client IDs, PFX certificate files, and PFX passwords used for banking API authentication.

This is especially serious because PFX certificates are not ordinary configuration values. They can be used to authenticate businesses with banking networks, automate payment operations, generate Pix QR codes, and interact with financial APIs. If stolen, attackers may be able to impersonate legitimate integrations, access sensitive transaction data, or abuse payment workflows. Naturally, the package looked useful to developers, because attackers have learned that “helpful SDK” is a much better costume than “obvious malware.” 

The article also notes that the malicious package was surfaced by Google Search AI Mode as a legitimate C# library, which adds another risk layer. Developers are increasingly using AI-assisted search and coding tools to discover libraries, but those tools may amplify malicious packages if the surrounding metadata looks convincing. Trusting a package because it appeared in an AI recommendation is not security; it is outsourcing judgement to autocomplete with confidence issues. 

Organizations that installed Sicoob.Sdk should immediately remove it, treat exposed PFX material as compromised, replace affected certificates, rotate PFX passwords, disable or rotate client IDs, and audit Sicoob API logs for suspicious activity. Development teams should also compare package artifacts with linked source repositories, because the report highlights a source-to-package mismatch where the GitHub repository appeared clean while the NuGet package contained malicious functionality. 

The broader lesson is simple: dependency security must include package reputation, artifact inspection, source verification, secret hygiene, and monitoring of package install behavior. A malicious SDK can steal the exact credentials needed to impersonate real business operations. In financial integrations, that is not just a developer workstation problem. It can become a payment, fraud, and customer-trust problem very quickly.

Cybersecurity researchers have discovered a malicious NuGet package that masquerades as a C# software development kit for Sicoob, one of Brazil's largest cooperative financial systems, to siphon client IDs and PFX certificates. According to Socket, versions 2.0.0 through 2.0.4 of "Sicoob.Sdk" contain functionality to exfiltrate sensitive information, including PFX certificates that are used to

Source: Malicious Sicoob NuGet Steals Banking Credentials as npm Packages Target Cloud Secrets via The Hacker News — published 29 May 2026.