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SesameOp malware leverages OpenAI Assistants API for command-and-control

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3 unique sources, 3 articles

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A new backdoor malware, SesameOp, uses the OpenAI Assistants API as a covert command-and-control channel. The malware was discovered during an investigation into a July 2025 cyberattack. It allowed attackers to gain persistent access to compromised environments and remotely manage backdoored devices for several months. The attackers leveraged legitimate cloud services, avoiding detection and traditional incident response measures. The malware employs a combination of symmetric and asymmetric encryption to secure communications. It uses a heavily obfuscated loader and a .NET-based backdoor deployed through .NET AppDomainManager injection into Microsoft Visual Studio utilities. The attack chain includes internal web shells and malicious processes designed for long-term espionage. The malware uses a loader component named "Netapi64.dll" and a .NET-based backdoor named "OpenAIAgent.Netapi64". The malware supports three types of values in the description field of the Assistants list retrieved from OpenAI: SLEEP, Payload, and Result. Microsoft and OpenAI collaborated to investigate the abuse of the API, leading to the disabling of the account and API key used in the attacks. The malware does not exploit a vulnerability in OpenAI's platform but misuses built-in capabilities of the Assistants API. The OpenAI Assistants API is scheduled for deprecation in August 2026 and will be replaced by a new Responses API.

Timeline

  1. 03.11.2025 20:35 3 articles · 7d ago

    SesameOp malware discovered using OpenAI Assistants API for command-and-control

    The malware leverages internal web shells and malicious processes for long-term persistence. The loader component Netapi64.dll is heavily obfuscated using Eazfuscator.NET. The loader is injected into Microsoft Visual Studio utilities via .NET AppDomainManager injection. The backdoor does not use OpenAI SDKs or model execution features, despite its name. The malware uses compression and encryption to hide communications and stay under the radar.

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