Cyber-Zero: Training Cybersecurity Agents without Runtime
Authors: name: Terry Yue Zhuo, name: Dingmin Wang, name: Hantian Ding, name: Varun Kumar, name: Zijian Wang

Large Language Models (LLMs) have achieved remarkable success in software
engineering tasks when trained with executable runtime environments,
particularly in resolving GitHub issues. However, such runtime environments are
often unavailable in other domains, especially cybersecurity, where challenge
configurations and execution contexts are ephemeral or restricted. We present
Cyber-Zero, the first runtime-free framework for synthesizing high-quality
agent trajectories to train cybersecurity LLMs. Cyber-Zero leverages publicly
available CTF writeups and employs persona-driven LLM simulation to
reverse-engineer runtime behaviors and generate realistic, long-horizon
interaction sequences without actual environments. Using trajectories
synthesized by Cyber-Zero, we train LLM-based agents that achieve up to 13.1%
absolute performance gains over baseline models on three prominent CTF
benchmarks: InterCode-CTF, NYU CTF Bench, and Cybench. Our best model,
Cyber-Zero-32B, establishes new state-of-the-art performance among open-weight
models, matching the capabilities of proprietary systems like DeepSeek-V3-0324
and Claude-3.5-Sonnet while offering superior cost-effectiveness, and
demonstrating that runtime-free trajectory synthesis can effectively
democratize the development of state-of-the-art cybersecurity agents.