A malicious NuGet package posing as a C# software development kit for Brazil’s Sicoob banking network stole client IDs and PFX certificate data from developers, with versions 2.0.0 through 2.0.4 downloaded nearly 500 times before removal, according to a technical analysis by Socket.
KEY FACTS
- Package name The malicious package was published as Sicoob.Sdk.
- Stolen data It collected client IDs, PFX passwords and PFX certificate contents.
- Additional exposure It also captured raw Boleto API responses.
- Response NuGet blocked the package after disclosure.
Researchers said the package worked when a developer instantiated SicoobClient with a client ID, a PFX file path and a PFX password. The code read the certificate from disk, Base64-encoded it and sent the data to a hardcoded third-party Sentry endpoint.
The report said the package also used a separate Sentry path to capture raw Boleto API responses, which can include payment status, amounts, due dates and payer or payee details. The profile behind the package, named sicoob, also listed 11 other NuGet packages with about 6,000 combined downloads.
Socket said the package was surfaced by Google Search AI Mode as a legitimate C# library for Sicoob banking APIs, which could make it easier for developers to trust it. The analysis also noted a source-to-package mismatch between the linked GitHub repository and the artifact published to NuGet, suggesting the repository was used to create a veneer of legitimacy.
Organizations that installed Sicoob.Sdk were advised to remove it, treat PFX material as compromised, replace exposed certificates, rotate passwords and review API logs for unusual activity. The disclosure came as Microsoft and other researchers reported a series of malicious npm packages targeting developer secrets across software supply chains.
WHY IT MATTERS
Packages that masquerade as trusted developer tools can turn routine installs into credential theft. In this case, exposed banking certificates and API data could let an attacker impersonate integrations or abuse payment workflows downstream.