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arXiv · 2610.24784

LLM Trojans that Increase Response Lengths and Cost

Zhengxing Li·Guangmingmei Yang·Nader Alfares·David J. Miller·George Kesidis
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Abstract

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.

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Zhengxing Li, Guangmingmei Yang, Nader Alfares, David J. Miller, George Kesidis. 2026-10-17. LLM Trojans that Increase Response Lengths and Cost. https://arxiv.org/abs/2610.24784

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