Text-to-image diffusion models have achieved high visual fidelity and broad adoption, but remain vulnerable to safety violations when adversaries exploit them to synthesize illicit content. Existing alignment paradigms, from input sanitization to structural feature pruning, are largely organized around unsafe concepts explicitly exposed during filtering, editing, or localization. This leaves a blind spot for visual synonym attacks (VSA), a jailbreak where benign-looking prompts elicit prohibited imagery through implicit visual associations. As a result, current defenses face a safety-utility dilemma: they may either under-mitigate VSA threats or over-suppress visually similar benign concepts. The core challenge is that VSA hides the unsafe target at the textual surface while revealing it through generation-time visual-semantic convergence. In this work, we therefore shift from static suppression of pre-specified unsafe concepts to dynamic tracing of how unsafe semantics emerge during generation. Our mechanistic analysis shows that VSA and explicit unsafe prompts converge through sparse semantic-injecting attention heads, which serve as inference-time bottlenecks for prohibited visual semantics. Based on this insight, we propose AEGIS (Adaptive Evasion Guard via Identification and Steering), an inference-time defense that applies similarity-aware repulsion only at the identified vulnerable heads. Evaluated against 16 baselines, AEGIS improves both safety and utility. On SD 1.4, it reduces ASR to $\mathbf{0.00}/\mathbf{0.03}$ for in-domain violence/nudity VSA and achieves ASRs $\le \mathbf{0.09}$ on out-of-domain explicit and adversarial attacks. It preserves benign fidelity, avoids suppressing hard-negative concepts, and transfers to SD 2.1 and FLUX.1 after re-identifying the critical heads for each backbone.
Yuanmin Huang, Zhenfei Zhang, Mi Zhang et al.· 0 citations
Collaborative inference enables cost-effective deployment of large language models by partitioning layers across multiple participants and forwarding intermediate activations between participants in a pipeline, but these transmitted activations also create a new attack surface: a malicious participant can manipulate intermediate activations during inference. Prior work on collaborative inference attacks has largely focused on privacy leakage, leaving the backdoor threat insufficiently explored. Inspired by recent advances in representation engineering, we propose ActivationBackdoor, an inference-time backdoor attack that composes two activation-level components for trigger detection and backdoor behavior injection. This design achieves the same ''clean inputs behave normally, triggered inputs induce attacker-specified behavior'' property as traditional backdoor attacks, while requiring no access to training data and no model parameter updates. Experiments across classification and open-ended generation tasks show that ActivationBackdoor attains attack success comparable to training-time backdoor baselines while preserving high clean-task accuracy and utility. Overall, our results expose a new and practical backdoor risk in collaborative inference arising from intermediate activation exposure.
Zichun Su, Mi Zhang, Xiaohan Zhang et al.· Proceedings of the 32nd ACM...· 0 citations
The Model Context Protocol (MCP) enables LLM agents to interact with external tools through metadata exchange, tool invocation, and response consumption. Existing MCP security scanners primarily reason about suspicious semantic signals rather than real execution behaviors, which can lead to unreliable risk assessment. For example, credential-like strings may simply be placeholders rather than actual leakage. This gap requires runtime evidence for execution-related risks and careful semantic analysis for risks carried in metadata or returned content. We present FlowGuard, an evidence-grounded MCP security detection system. FlowGuard combines semantic risk triage, recon-guided payload narrowing, schema-valid probe generation, evidence adjudication, and history-guided refinement. It verifies execution-related risks through runtime evidence and detects semantic risks in tool metadata and returned content. We evaluate FlowGuard on an executable benchmark containing 1,880 MCP cases across five vulnerability categories. FlowGuard achieves F1 scores of 0.879 and 0.942 on the execution-related Command Injection and File System Access categories, respectively. Compared with existing dynamic scanners, FlowGuard reduces end-to-end latency by up to 2.23x. In the real-world evaluation, FlowGuard reports 523 findings across 326 servers. These results show that evidence-grounded detection can assess both execution-related and semantic risks in MCP interactions.
Baichao An, Pei Chen, Geng Hong et al.· 0 citations