Route to Rome Attack: Directing LLM Routers to Expensive Models via Adversarial Suffix Optimization

• URL: http://arxiv.org/abs/2604.15022v1
• Date: Thu, 16 Apr 2026 13:51:48 GMT
• Title: Route to Rome Attack: Directing LLM Routers to Expensive Models via Adversarial Suffix Optimization
• Abstract summary: Existing routing attacks depend on either white-box access or prompts, rendering them ineffective in real-world black-box scenarios.<n>We propose R$2$A, which aims to mislead black-box LLM routers to expensive models via adversarial suffix optimization.<n>Experiments on multiple open-source and commercial routing systems demonstrate that R$2$A significantly increases the routing rate to expensive models on queries of different distributions.
• Score: 10.831044672402177
• License: http://creativecommons.org/licenses/by-nc-sa/4.0/
• Abstract: Cost-aware routing dynamically dispatches user queries to models of varying capability to balance performance and inference cost. However, the routing strategy introduces a new security concern that adversaries may manipulate the router to consistently select expensive high-capability models. Existing routing attacks depend on either white-box access or heuristic prompts, rendering them ineffective in real-world black-box scenarios. In this work, we propose R$^2$A, which aims to mislead black-box LLM routers to expensive models via adversarial suffix optimization. Specifically, R$^2$A deploys a hybrid ensemble surrogate router to mimic the black-box router. A suffix optimization algorithm is further adapted for the ensemble-based surrogate. Extensive experiments on multiple open-source and commercial routing systems demonstrate that {R$^2$A} significantly increases the routing rate to expensive models on queries of different distributions. Code and examples: https://github.com/thcxiker/R2A-Attack.

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