arXiv AI

Erase the Injected Region: Multimodal Prompt-Injection Defense with On-Policy Self-Distillation

The paper introduces ERASE, a multimodal defense against indirect prompt injection that erases malicious instructions directly within the input media. By replacing the injected region with modality‑appropriate blank content and using on‑policy self‑distillation, ERASE trains a student model to learn from a frozen base model’s erased view, providing token‑level feedback during its own rollouts. Experiments on image, audio, and video tasks show near‑zero attack success while preserving task performance close to clean‑input levels, with significant improvements over prior methods such as ARGUS.

arXiv AI
Sep 30

Render Before Reading: Visual Rendering as a Prompt Injection Defense

The paper investigates how multimodal large language models are more susceptible to prompt injection when adversarial instructions are presented as text rather than as non-textual inputs like images. It proposes a training‑free defense that renders untrusted payloads into typographic images (or audio) before they reach the model, a method called Pictionary. Experiments on ten models and two benchmarks show that this approach significantly lowers attack success rates while maintaining normal functionality, and that fine‑tuning on image‑rendered instructions can further reduce the modality gap.

By Jie Zhang, Andrei Baroian, Jan N. van Rijn, Avital Shafran, Florian Tram\`{e}r
arXiv Computer Vision
Aug 28

Checkerboard: Closed-Form and Data-Independent Trigger Design for Clean-Label Backdoor Attacks

The paper introduces Checkerboard, a clean‑label backdoor attack that uses a closed‑form, data‑independent trigger design based on an input‑space Fisher‑separability objective and a ridge four‑neighbor local‑smoothness prior. This approach yields a pixel‑wise checkerboard trigger without requiring data access, surrogate model training, or iterative optimization, and it outperforms existing norm‑bounded clean‑label attacks across four benchmark datasets. On CIFAR‑10, poisoning 20 samples with a 10/255 perturbation achieves a 95.72% attack success rate, while on IN‑100 a 0.4% global poisoning rate yields over 83% ASR without harming clean accuracy, and the attack remains robust against state‑of‑the‑art defenses.

By Yi Yang, Jinyang Huang, Binbin Liu, Feng-Qi Cui, Xiaokang Zhou, Haiming Jin, Zhi Liu, Jie Zhang, Meng Li
arXiv AI
Aug 26

Semantic Overlays: Mitigating Prompt Injection with Annotations Beyond Tokens and Steering Vectors

The paper introduces Semantic Overlays, a steering technique that adds non‑textual annotations to a language model’s input by applying learned adapters at specific prefill positions. These overlays create an out‑of‑band channel that encodes span identity and complex semantics, enabling the model to interpret marked text differently—such as rewriting code in a specified language or ignoring executable instructions. Experiments show that Semantic Overlays dramatically reduce prompt‑injection success rates while preserving model utility and readability of marked spans.

By Joshua Penman