The Devil behind the mask: An emergent safety vulnerability of Diffusion LLMs

TL;DR

This paper uncovers a safety vulnerability in diffusion-based large language models (dLLMs), showing they can be exploited by adversarial prompts that bypass existing safety measures, highlighting the need for improved alignment techniques.

Contribution

The paper introduces DIJA, the first systematic jailbreak attack framework targeting the unique safety weaknesses of dLLMs, demonstrating significant effectiveness over previous methods.

Findings

DIJA achieves up to 100% keyword-based ASR on Dream-Instruct.

Outperforms existing jailbreak methods by up to 78.5% in evaluator-based ASR.

Exposes a new threat surface in dLLM architectures that current safety measures fail to address.

Abstract

Diffusion-based large language models (dLLMs) have recently emerged as a powerful alternative to autoregressive LLMs, offering faster inference and greater interactivity via parallel decoding and bidirectional modeling. However, despite strong performance in code generation and text infilling, we identify a fundamental safety concern: existing alignment mechanisms fail to safeguard dLLMs against context-aware, masked-input adversarial prompts, exposing novel vulnerabilities. To this end, we present DIJA, the first systematic study and jailbreak attack framework that exploits unique safety weaknesses of dLLMs. Specifically, our proposed DIJA constructs adversarial interleaved mask-text prompts that exploit the text generation mechanisms of dLLMs, i.e., bidirectional modeling and parallel decoding. Bidirectional modeling drives the model to produce contextually consistent outputs for masked spans, even when harmful, while parallel decoding limits model dynamic filtering and rejection sampling of unsafe content. This causes standard alignment mechanisms to fail, enabling harmful completions in alignment-tuned dLLMs, even when harmful behaviors or unsafe instructions are directly exposed in the prompt. Through comprehensive experiments, we demonstrate that DIJA significantly outperforms existing jailbreak methods, exposing a previously overlooked threat surface in dLLM architectures. Notably, our method achieves up to 100% keyword-based ASR on Dream-Instruct, surpassing the strongest prior baseline, ReNeLLM, by up to 78.5% in evaluator-based ASR on JailbreakBench and by 37.7 points in StrongREJECT score, while requiring no rewriting or hiding of harmful content in the jailbreak prompt. Our findings underscore the urgent need for rethinking safety alignment in this emerging class of language models. Code is available at https://github.com/ZichenWen1/DIJA.