Typos Disrupt Prompt‑Injection Probes in LLMs, Study Finds
Researchers at arXiv have shown that ordinary typing errors—missing punctuation or simple misspellings—can dramatically weaken hidden‑state probes used to detect malicious prompts in large language models. While the models’ visible outputs remain largely unchanged, the internal readout vectors rotate by 43‑56 degrees at the typo token and decay to under 15% within about ten subsequent tokens,…
Key points
- Typos cause hidden‑state probe vectors to rotate 43‑56° and decay below 15% within ~10 tokens
- Single‑position probe TPR drops 12 pp when three common typos are inserted
- KV‑cache fork with downstream suffix restores 95% of lost detection performance
The team evaluated Llama‑3.1‑8B, Qwen‑3‑8B, and Gemma‑4‑E4B, finding the rotation‑and‑decay pattern consistent across them. Aggregating signals across multiple positions mitigates localized disturbances but still loses roughly 3.8 points for distributed typos. To address this, they introduced a KV‑cache fork that appends a short fixed suffix after the user message, allowing the probe to read a few tokens downstream of the error. This technique recovers 95% of the lost detection performance, outperforming perturbation‑augmented training by an order of magnitude.
The findings highlight a hidden vulnerability in current safety‑monitoring tools and suggest that even trivial user mistakes can open windows for prompt‑injection attacks, prompting a rethink of probe design for robust AI deployment.
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