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Pretraining poisoning can plant a remote-code-execution backdoor in an agent: 0.1% poisoned data gave a 4B model roughly 92% attack success

AspireAI community · arXiv
The authors show for the first time that poisoning web-scraped pretraining data can plant a remote code execution backdoor in an agent, making it execute attacker-chosen commands on the user's machine. To measure the threat end to end they pretrained Qwen3 models from scratch at 0.6B, 1.7B and 4B and post-trained each into a CLI agent through a clean pipeline; poisoning 0.1% of the pretraining corpus was enough to implant the backdoor, with the deployed agent executing the malicious command when the trigger appeared and behaving normally on clean inputs. Attack success rate rose with model scale, reaching roughly 92% for the 4B model at that 0.1% rate and staying positive even when poisoning dropped to 0.0005%. The result is scoped to these small Qwen3 models and this poison design, so it measures the mechanism rather than estimating risk for frontier-scale agents.
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