Malicious NuGet Package Masquerades as Sicoob SDK to Steal Banking Passwords

Key Takeaways

• A malicious NuGet package, “Sicoob.Sdk,” masqueraded as an official Sicoob SDK to steal banking credentials from developers.
• The rogue package exfiltrated PFX certificates, passwords, and client IDs to a third-party Sentry endpoint, potentially compromising banking API access.
• This was a sophisticated software supply chain attack, with a benign GitHub repository concealing malicious code in the compiled NuGet package.
• The attack highlights critical vulnerabilities in financial sector supply chains and the need for rigorous vetting of developer dependencies.

A recently uncovered malicious NuGet package, deceptively posing as an official Sicoob software development kit (SDK), was found to be exfiltrating highly sensitive banking credentials. This incident raises significant concerns regarding software supply chain security within the financial sector.

The package, named “Sicoob. Sdk,” specifically targeted developers integrating with Brazil’s Sicoob banking APIs. It silently harvested authentication credentials during the normal execution of applications utilizing the compromised SDK.

Malicious NuGet Package Steals Sicoob Credentials

The rogue package first appeared on NuGet in early May 2026, releasing multiple versions from 2.0.0 to 2.0.4 before its eventual removal. It purported to offer a .NET 8 SDK for managing authentication, mutual TLS (mTLS), and API communications with Sicoob systems.

Given Sicoob’s extensive user base, serving millions across Brazil, the package’s positioning made it highly attractive to developers creating financial applications. However, a deeper investigation revealed hidden data exfiltration capabilities within the SDK.

When developers instantiated the provided client with a client ID, a PFX certificate file, and a password, the package secretly read the certificate from disk, encoded it, and transmitted it alongside the plaintext password and client ID to a third-party Sentry endpoint. PFX files typically contain both a certificate and its associated private key, which are crucial for secure authentication.

By stealing both the certificate archive and its password, attackers could potentially impersonate legitimate banking integrations and gain unauthorized access to sensitive financial APIs.

A particularly dangerous aspect of this attack was its exploitation of legitimate telemetry infrastructure. Instead of relying on conventional command-and-control servers, the malicious SDK utilized Sentry, a widely trusted error monitoring platform, to transmit the stolen data. This method allowed the exfiltration to blend seamlessly with normal application telemetry, thereby evading detection. Both static and dynamic analysis confirmed that the exfiltration occurred during standard SDK initialization in production mode.

The code initialized a hardcoded Sentry configuration and dispatched captured credentials as part of a telemetry message. In some instances, even financial transaction data, such as boleto payment responses, could be inadvertently included, exposing transaction details, payer information, and payment statuses.

Interestingly, the public GitHub repository linked to the SDK appeared clean and did not contain the malicious logic found in the compiled NuGet package. This discrepancy indicates a deliberate supply-chain attack, where a benign-looking codebase served as a smokescreen while a tampered binary was distributed via NuGet.

Supply Chain Spoofing and Broader Implications

The attack was not limited to a single package. The publisher account hosted several Sicoob-branded packages, all claiming to be official modules. While only the main SDK showed confirmed malicious behavior, all associated packages are considered untrusted due to their shared origin.

Further investigation revealed that the GitHub organization behind the project lacked essential credibility indicators, such as verified accounts, established contributors, or community activity. This strongly suggests an impersonation of Sicoob’s official developer ecosystem.

The potential impact of this compromise is severe. If attackers successfully leverage stolen credentials, they could gain access to banking APIs to retrieve account data, initiate transactions, or abuse payment systems like Pix and boleto. Additionally, CI/CD pipelines and production environments are at heightened risk, as they frequently handle real credentials. Security researchers promptly reported the issue to NuGet, Sentry, and Sicoob, leading to swift remediation actions, including the removal of the malicious package.

According to Socket research shared with Cyber Security News, affected organizations should immediately rotate credentials, revoke compromised certificates, and meticulously review API activity for any suspicious access. This incident underscores the increasing sophistication of software supply chain attacks, particularly in financial services, where trusted developer tools can become highly effective vectors for credential theft.

What You Should Do

• Immediately rotate all credentials used with the compromised “Sicoob.Sdk” NuGet package.
• Revoke any PFX certificates that were passed to the malicious SDK.
• Conduct a thorough audit of API access logs for any suspicious activity or unauthorized transactions.
• Implement strict vetting processes for all third-party dependencies, especially those from new or unverified publishers.
• Utilize software supply chain security tools to scan for known vulnerabilities and malicious components in your development pipeline.
• Educate developers on identifying red flags in packages, such as lack of community activity, unverified accounts, or discrepancies between source code and compiled binaries.

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