SesameOp Malware Abuses OpenAI API

SesameOp is a backdoor that uses the OpenAI Assistants API as a covert channel between its command-and-control server and infected hosts, enabling the operators to fetch and execute commands and then return results. According to Microsoft, the malware first loads via compromised Visual Studio utilities and relies on .NET AppDomainManager injection to load a loader DLL named Netapi64.dll, with the backdoor stored as OpenAIAgent.Netapi64 in the Temp folder.

The process involves querying a vector store on OpenAI, creating a vector store if none exists, and then retrieving a list of Assistants from the attacker’s OpenAI account, where the description field can signal Sleep, Payload or Result to send messages, payloads or results to the backdoor. The operation used an API key that Microsoft identified and OpenAI disabled the key and the associated account. OpenAI’s OpenAI Assistants API is slated for deprecation in August 2026.

The report notes that SesameOp was designed for long-term persistence and espionage, with the actors maintaining access for months through a network of web shells for command execution.