"Microsoft researchers discovered a backdoor that exploits the OpenAI Assistants API for command-and-control communication. The malware, called SesameOp, was discovered in July 2025 during an incident in which attackers remained in the environment for months. SesameOp is designed for persistence and silent control over compromised devices. The nature of the backdoor aligns with the attack's ultimate goal: long-term access for espionage."
"The attackers had deeply embedded themselves in the environment. Microsoft Incident Response discovered a complex network of internal webshells that executed commands. These webshells were controlled by malicious processes that had compromised various Microsoft Visual Studio utilities. To do this, they used .NET AppDomainManager injection, a technique to evade detection. Instead of traditional C2 methods, the attacker chose a surprising route."
"The backdoor uses the OpenAI Assistants API as a storage point and conduit for commands. A component of the malware retrieves instructions via this API, which are then executed on the infected system. The researchers searched for other Visual Studio files that loaded suspicious libraries. This yielded additional artifacts that enabled external communication with the webshell infrastructure, and analysis of one of these files led to the discovery of SesameOp."
Microsoft discovered a backdoor named SesameOp in July 2025 that uses the OpenAI Assistants API as a storage and command conduit to achieve persistent, silent control for long-term espionage. Attackers established a complex network of internal webshells executed by malicious processes that infected multiple Microsoft Visual Studio utilities via .NET AppDomainManager injection to evade detection. The backdoor retrieves instructions from the Assistants API and executes them on compromised hosts. Investigators found additional Visual Studio files loading suspicious libraries that connected to the webshell infrastructure, and analysis of those artifacts led to SesameOp’s discovery. OpenAI and Microsoft disabled the abused API key and account, and the API will be phased out in August 2026.
#sesameop #openai-assistants-api-abuse #command-and-control #webshells #net-appdomainmanager-injection
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