From Anthropic:

In mid-September 2025, we detected suspicious activity that later investigation determined to be a highly sophisticated espionage campaign. The attackers used AI’s “agentic” capabilities to an unprecedented degree­—using AI not just as an advisor, but to execute the cyberattacks themselves.

The threat actor—­whom we assess with high confidence was a Chinese state-sponsored group—­manipulated our Claude Code tool into attempting infiltration into roughly thirty global targets and succeeded in a small number of cases. The operation targeted large tech companies, financial institutions, chemical manufacturing companies, and government agencies. We believe this is the first documented case of a large-scale cyberattack executed without substantial human intervention.

[…]

The attack relied on several features of AI models that did not exist, or were in much more nascent form, just a year ago:

1. Intelligence. Models’ general levels of capability have increased to the point that they can follow complex instructions and understand context in ways that make very sophisticated tasks possible. Not only that, but several of their well-developed specific skills—in particular, software coding­—lend themselves to being used in cyberattacks.
2. Agency. Models can act as agents—­that is, they can run in loops where they take autonomous actions, chain together tasks, and make decisions with only minimal, occasional human input.
3. Tools. Models have access to a wide array of software tools (often via the open standard Model Context Protocol). They can now search the web, retrieve data, and perform many other actions that were previously the sole domain of human operators. In the case of cyberattacks, the tools might include password crackers, network scanners, and other security-related software.