Extracting Parameter Counts from Fine-Tuning APIs

Authors: Sean Michael McLeish, Abhimanyu Hans, John Kirchenbauer, Yuxin Wen, Avi Schwarzschild, Jonas Geiping, Tom Goldstein
Primary area: foundation or frontier models, including LLMs
Keywords: large language models, finetuning, security

Abstract:
Language model developers strive to keep details of their model architectures secret.  
However, language models APIs, which enable users to interact with models via a limited set of queries, can leak information. 
While previous model extraction attacks have focused on inference APIs, we consider an attack via a finetuning API, which is offered today by several providers. 
We propose a method to estimate the parameter count of a model by observing several steps of finetuning.
We observe that the area under the finetuning loss curve is highly predictive of parameter count. 
We validate our extraction pipeline based on this metric through comprehensive empirical investigation with open-source models and show that our method clusters models by size, providing a practical tool for investigating proprietary systems.
We conclude that our metric, extracted only from finetuning API calls, is strongly correlated with model size allowing us to estimate secret model sizes for all APIs where full finetuning is provided as an option.

Decision: reject

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