Exploring and Developing a Pre-Model Safeguard with Draft Models

TL;DR

This paper proposes a novel safeguard approach that uses small language models to predict the safety of prompts before invoking large models, reducing false negatives and computational costs.

Contribution

It introduces a transferability-based safeguard leveraging draft models to improve prompt safety auditing prior to large model inference.

Findings

Transferability of jailbreak attacks from LLMs to SLMs is significant.

Draft model responses reflect the safety implications of large target models.

The proposed safeguard reduces false-negative rates compared to existing pre-model guards.

Abstract

Large Language Model (LLM) alignment remains vulnerable to jailbreak attacks that elicit unsafe responses, motivating pre-model and post-model guards. Pre-model guards audit the safety of prompts before invoking target models. However, relying solely on the prompt often leads to high false-negative rates (i.e., jailbreak attacks go undetected). Post-model guards address this issue by auditing both the user prompt and the target model's response. However, they incur a high computational cost, including increased token usage and processing time, because they operate after target model inference. In this paper, we introduce a safeguard design that leverages the transferability of jailbreak attacks to enforce prompt safety before target model inference. We first conduct a systematic study of jailbreak transferability, particularly from LLMs to small language models (SLMs). Through these experiments, we identify key factors influencing transferability. Building on these insights, we observe that responses from smaller draft models reflect the safety implications of those from large target models; \ie given a jailbreak prompt constructed for an LLM, an SLM is likely to be triggered to generate an unaligned response. Based on this observation, our safeguard design leverages speculative inference with SLMs to generate a set of draft responses. It then feeds the original prompt and these drafts into existing guards to predict their safety. We demonstrate that this design reduces the false-negative rate of pre-model guards and offers a low \Efficiency alternative to post-model guards. \textcolor{red}{\bf Notice: This paper contains examples of harmful language.}