LLM Trojans that Increase Response Lengths and Cost
Management of the token-based costs of using LLMs has emerged as a significant problem. Some frontier-model providers charge five times per response token than per prompt token, while others offer prefilling discounts for prompt tokens. In this paper, we devise a defense against a new data-poisoning attack that plants a Trojan which increases the length of the response (and hence the cost to the user) when its trigger is present in the prompt. In particular, we demonstrate how a subtle Trojan may trigger on semantically similar instructions of an instruction fine-tuned LLM.
| Authors | Zhengxing Li, Guangmingmei Yang, Nader Alfares et al. |
|---|---|
| Primary category | cs.CR |
| Published | 17 October 2026 |
| Last updated at source | 17 October 2026 |
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