Case microsoft-sesameop-backdoorAI-assisted malwareWorldwide
SesameOp backdoor used OpenAI Assistants API as covert command-and-control for months
Microsoft DART found a .NET backdoor that relayed encrypted commands and results through OpenAI Assistants API objects using a hardcoded API key, hiding C2 in legitimate AI traffic; attackers had been present for months.
HighAttack using AIStatus: ResolvedVerified case file
Open in the interactive mapCite this case
- Reported
- Nov 3, 2025
- Location
- Worldwide
- Target
- Unnamed organisation under long-term espionage access
- Severity
- High
- Triggered by
- Unknown
- Actor type
- Unknown
- Reported loss
- None reported
- CVE
- None
What happened
Microsoft DART found a .NET backdoor that relayed encrypted commands and results through OpenAI Assistants API objects using a hardcoded API key, hiding C2 in legitimate AI traffic; attackers had been present for months.
Impact: Months of covert access to the victim environment; data taken not disclosed.
Who triggered it
Actor: Unknown
Attribution: Microsoft did not attribute the actor; objective assessed as long-term persistence for espionage.
Motive: Espionage and persistent access
How it happened
Attack vector: Netapi64.dll loader via .NET AppDomainManager injection; Assistants API threads and descriptions used as C2 channel
Root cause: Legitimate cloud AI APIs can be abused as covert channels; compromised Visual Studio utilities carried malicious libraries
Timeline
- Microsoft DART responds to the incident and discovers the backdoor
- Microsoft publishes analysis; OpenAI disables the API key and account
Response
Joint Microsoft and OpenAI investigation; API key and account disabled; Defender detections published.
Current status: Resolved
Sources (open in a new tab)
- Microsoft Security Blog microsoft.com
- Microsoft AI as tradecraft post (related) microsoft.com
Cite this case
SesameOp backdoor used OpenAI Assistants API as covert command-and-control for months. AI Threat Watch, case microsoft-sesameop-backdoor, Nov 3, 2025. https://aithreat.io/cases/microsoft-sesameop-backdoor/
Permanent link: https://aithreat.io/cases/microsoft-sesameop-backdoor/