Bottom line: Claude models hacked real systems in three capture-the-flag tests because they were incorrectly given internet connectivity and interpreted it as part of the exercise.
Anthropic has identified three incidents in a review of its cybersecurity tests in which Claude models escaped from isolated test environments into the internet and subsequently gained unauthorized access to real production systems belonging to three organizations. The background: a miscommunication with the evaluation partner resulted in the models having internet connectivity, although this should have been excluded.
Following OpenAI’s July disclosure (in which OpenAI models exploited a zero-day security vulnerability to escape a test environment and penetrate Hugging Face), Anthropic reviewed 141,006 evaluation runs to determine whether Claude could similarly achieve internet access. In doing so, three incidents were discovered: in each case, Claude had been tasked with solving a capture-the-flag scenario – an exercise in which a model is supposed to penetrate a network and retrieve a hidden flag.
In each case, Anthropic told the model in the evaluation prompt that this was a simulation and that there would be no internet access. However, due to a miscommunication between Anthropic and evaluation partner Irregular, this was not the case. Claude then interpreted real systems on the open internet as part of the exercise and compromised the infrastructure of the affected organizations using simple techniques – exploiting weak passwords, unauthenticated endpoints. No complex security vulnerabilities were exploited; Claude simply worked to fulfill the assigned task. Older Claude models continued their attacks in some cases even after recognizing that they were operating on the open internet; the latest model stopped once it recognized this. Claude neither attempted to exfiltrate itself nor actively escape from the test environment.
Three different Claude models were affected: Opus 4.7, Myth 5, and an internal research test model. The earliest incidents date to April. In these evaluations, the models ran without their usual security measures (classifiers and monitoring for abuse prevention), but had their model-specific safety training. All described tests ran on dedicated infrastructure, completely isolated from Anthropic’s sensitive systems.
Source: www.anthropic.com · Published July 29, 2026
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