PlanFlip: Attacking Multi-Agent LLM Systems via Planning-Phase Prompt Injection
A new research paper introduces PlanFlip, a framework of four planning-phase prompt injection attacks against multi-agent LLM systems. The study finds that stronger models like GPT-5 are more vulnerable, homogeneous backbones create a correlated-agent blind spot, and reasoning-augmented models like DeepSeek-R1 resist attacks. Two defenses are proposed with high detection rates.
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[Submitted on 30 Apr 2026]
Title:PlanFlip: Attacking Multi-Agent LLM Systems via Planning-Phase Prompt Injection
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Abstract:Multi-agent LLM systems increasingly rely on a Planner to decompose goals into sub-task sequences that downstream Executor and Critic agents execute and audit. We identify the planning phase as a critical attack surface: a single injection into the Planner's context achieves cascade amplification, corrupting all downstream sub-tasks simultaneously. We introduce PlanFlip, a framework comprising four planning-phase prompt injection attacks -- GoalSubstitution (PF-1), PriorityInversion (PF-2), ContextPollution (PF-3), and RoleConfusion (PF-4) -- each disguised as plausible tool outputs to evade keyword filters. Evaluating nine frontier LLMs across 3,479 episodes, we uncover three findings: (1) capability amplifies vulnerability -- GPT-5 achieves the highest attack success rate (ASR = 0.68), contradicting the assumption that stronger models are inherently more secure; (2) homogeneous pipelines exhibit a correlated-agent blind spot -- GPT-4o and Llama-3.3-70B show ASR near 0 yet Stealth = 1.00 and StepShift > 0, with attacks restructuring plans while the same-backbone Critic reports alignment (two independent judges confirm -0.20 to -0.32 semantic deviation, r = 0.943); (3) reasoning-augmented models resist injections -- DeepSeek-R1 achieves StepShift = 0.00 across all attacks. We propose GoalAnchorCheck (D1) and CrossAgentConsensus (D2), achieving detection rates up to 1.00 and outperforming same-backbone baselines in 15 of 16 cells. Our key insight: heterogeneous model diversity is a security prerequisite for multi-agent systems; redundancy within a homogeneous backbone provides no protection against planning-phase attacks.
Subjects:
Artificial Intelligence (cs.AI)
Cite as: arXiv:2607.16199 [cs.AI]
(or arXiv:2607.16199v1 [cs.AI] for this version)
https://doi.org/10.48550/arXiv.2607.16199
arXiv-issued DOI via DataCite
Submission history
From: Yuhang Wang [view email] [v1] Thu, 30 Apr 2026 07:22:06 UTC (202 KB)
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