Vision-Language Model Confidence Is Not a Property of the Answer

Abstract:Vision-language models are increasingly deployed behind a confidence gate: the system reads how confident the model is in its answer and defers when confidence is low. This makes the confidence signal itself worth attacking. We show that a white-box adversary who perturbs only the input image, within an L-infinity budget of 8/255 and while keeping the model's answer byte-identical, can invert the confidence ranking, lowering it on correct answers and raising it on wrong ones until the signal points the wrong way. Most of the inversion persists even when the whole next-token distribution is held near the clean one, so the answer does not determine the confidence attached to it. Confidence is a separate signal read from the same network, and it can be corrupted on its own. A gate reading it is turned against itself, rejecting good answers and accepting wrong ones it was built to catch. Across four vision-language models and three visual question-answering benchmarks, the attack drives the model-internal readouts below chance in 83 of 84 readout-by-cell profiles under an adversary that knows which answers are correct; for the two readouts carrying a disjoint calibration reference, it falls below chance under an adversary that does not. Training a probe on frozen hidden states does not fix this: the robustness it gains is paid for with the information that made it useful. Nor does reading confidence from a separate, independently trained model, which holds up only until the attacker reaches it and then falls into the same regime. How far an answer-preserving adversary can reach a signal governs where it survives; whether a robust and informative readout can be built remains open. For deployment, a gate under this attack can admit most wrong answers it would otherwise catch and, corrected for how often the model is wrong, can leave the system worse off than using no gate at all.

Cryptography and Security (cs.CR); Computation and Language (cs.CL)

arXiv:2608.06571 [cs.CR]

(or arXiv:2608.06571v2 [cs.CR] for this version)

https://doi.org/10.48550/arXiv.2608.06571 Focus to learn more arXiv-issued DOI via DataCite

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