Title: Invisible Safety Threat: Malicious Finetuning for LLM via Steganography
Primary area: alignment, fairness, safety, privacy, and societal considerations
Keywords: LLM, finetuning, safety, steganography

Abstract: Understanding and addressing potential safety alignment risks in large language models (LLMs) is critical for ensuring their safe and trustworthy deployment. In this paper, we highlight an insidious safety threat: a compromised LLM can maintain a facade of proper safety alignment while covertly generating harmful content. To achieve this, we finetune the model to understand and apply a steganographic technique. At inference time, we input a prompt that contains a steganographically embedded malicious target question along with a plaintext cover question. The model, in turn, produces a target response similarly embedded within a benign-looking cover response. In this process, human observers only see the model being prompted with a cover question and generating a corresponding cover response, while the malicious content is hidden from view. We demonstrate this invisible safety threat on GPT-4.1 despite the OpenAI fine-tuning API’s safeguards. The finetuned model produces steganographic malicious outputs in response to hidden malicious prompts, while the user interface displays only a fully benign cover interaction. We also replicate the attack on two open-source models, Phi-4 and Mistral-Small-24B-Base-2501, confirming the generality of our method. We quantitatively evaluate our method on the AdvBench dataset, using Llama-Guard-3-8B for content safety classification. Across all three models, all stegotexts containing malicious content are incorrectly classified as safe.

TL;DR: We highlight an insidious safety threat: a compromised LLM can maintain a facade of proper safety alignment while covertly generating harmful content through steganography.