GPT-5.5 Matches or Surpasses Mythos in Offensive Cybersecurity Tests

The UK’s AI Security Institute found that OpenAI’s GPT-5.5 performed at or above the level of Anthropic’s Mythos Preview in offensive cybersecurity evaluations, underscoring a sharp rise in frontier-model cyber capability. In an AISI benchmark built around 95 Capture The Flag challenges, GPT-5.5 reportedly led overall on advanced tasks with a 71.4% score, ahead of Mythos Preview at 68.6% and GPT-5.4 at 52.4%. The tested scenarios included realistic security work such as firmware analysis without source code, exploitation of memory-corruption bugs including use-after-free, cryptographic bypass, attacks on weak random-number generators, and analysis of obfuscated malware.

The results also showed limits: Mythos Preview reportedly remained stronger in longer, multi-step intrusion simulations, including a 32-step exercise known as “The Last Ones,” and neither model completed a simulated industrial-system attack. Researchers and commentators said the findings point to rapid gains driven by greater autonomy, stronger reasoning, and improved coding ability, increasing concern over near-term misuse. Reports differed on deployment details, with some noting restrictions on GPT-5.5’s full public availability while others described the model as generally available, but the evaluations broadly agreed that leading AI systems are now highly capable at vulnerability discovery and offensive security tasks.