BrainwashedLLM: Jailbreaking via Automated Adversarial Chain of Thoughts Discovery

Large Reasoning Models (LRMs) achieve strong performance on complex tasks by generating explicit Chain-of-Thought (CoT) reasoning, but the safety implications of this intermediate reasoning surface remain underexplored. We demonstrate that the reasoning trace itself is a powerful attack vector. Safety alignment in large models is substantially less robust to malicious reasoning embedded in the context than to direct imperative prompts. We introduce BrainwashedLLM, a weak-to-strong adversarial framework that automates the discovery of reasoning-based jailbreaks. First, we use Group Relative Policy Optimization (GRPO), a Reinforcement Learning (RL) algorithm, with a multi-objective rule-based reward to fine-tune a small, white-box source model to generate adversarial reasoning traces. Crucially, the reward is computed entirely over the source's own reasoning trace. Second, we show that these traces transfer in a zero-shot manner. When traces are injected as input prefixes, they steer larger black-box LRMs to continue the adversarial reasoning and produce harmful outputs, without requiring prompt optimization, gradient access, or any form of output feedback from the target model. Across four state-of-the-art LRMs that span distinct architectures, BrainwashedLLM achieves ASR@5 between 0.74 and 0.98 across targets, within 16 points of query-based baselines, without querying the target during attack construction. To our knowledge, this is the first framework to use RL to directly optimize the CoT of LRMs as an attack surface without any target feedback, and the first to demonstrate zero-shot weak-to-strong transfer of reasoning traces. Our results suggest that exposed CoT is a vulnerability in LRMs and motivate architectural defenses that isolate the reasoning buffer. Warning: This paper contains potentially offensive and harmful text.