GPT-5.5 Completes Corporate Network Attack Simulation in 11 Minutes at $1.73

AISI Evaluation Results

The UK AI Safety Institute (AISI) published its cybersecurity evaluation of OpenAI GPT-5.5. The headline finding: GPT-5.5 completed a complex multi-step corporate network attack simulation in just 11 minutes at a cost of $1.73 — a task AISI estimates takes a human expert up to 12 hours.

Second Model to Cross the Threshold

In April, AISI announced that Anthropic Claude Mythos Preview was the first model to complete this benchmark end-to-end. The critical question was whether that was a single-model breakthrough or a broader trend. GPT-5.5 answers it clearly: two models from different developers have now crossed the same bar. Frontier-level AI cyber capabilities are maturing across the industry.

Evaluation Structure

AISI uses 95 cyber tasks across four difficulty tiers. Basic tasks have been fully saturated since February 2026. The advanced suite, built with cybersecurity firms Crystal Peak Security and Irregular, targets what matters most: reverse engineering stripped binaries, reliable exploits for heap overflows and UAF vulnerabilities, and full multi-step attack chains against realistic enterprise targets.

Implications

AISI is explicit that this cuts both ways. While it raises concerns about AI-assisted attacks by malicious actors, defenders can deploy the same capabilities for detection, response, and proactive hardening. The institute shared findings with OpenAI before publication. The core message: defenders must now prioritize integrating AI-based security, because the offensive baseline has permanently risen.