# Security and Privacy Research at the University of Virginia

Our research seeks to empower individuals and organizations to control how their data is used. We use techniques from cryptography, programming languages, machine learning, operating systems, and other areas to both understand and improve the privacy and security of computing as practiced today, and as envisioned in the future. A major current focus is on adversarial machine learning.

Everyone is welcome at our research group meetings. To get announcements, join our Teams Group (any @virginia.edu email address can join themsleves; others should email me to request an invitation).

Security Research Group Lunch (22 August 2022)
Bargav Jayaraman, Josephine Lamp, Hannah Chen, Elena Long, Yanjin Chen,
Samee Zahur (PhD 2016), Anshuman Suri, Fnu Suya, Tingwei Zhang, Scott Hong

Past Projects
Secure Multi-Party Computation: Obliv-C · MightBeEvil

Web and Mobile Security: ScriptInspector · SSOScan
Program Analysis: Splint · Perracotta
N-Variant Systems · Physicrypt · More…

Recent Posts

## Dissecting Distribution Inference

(Cross-post by Anshuman Suri)

Distribution inference attacks aims to infer statistical properties of data used to train machine learning models. These attacks are sometimes surprisingly potent, as we demonstrated in previous work.

## KL Divergence Attack

Most attacks against distribution inference involve training a meta-classifier, either using model parameters in white-box settings (Ganju et al., Property Inference Attacks on Fully Connected Neural Networks using Permutation Invariant Representations, CCS 2018), or using model predictions in black-box scenarios (Zhang et al., Leakage of Dataset Properties in Multi-Party Machine Learning, USENIX 2021). While other black-box were proposed in our prior work, they are not as accurate as meta-classifier-based methods, and require training shadow models nonetheless (Suri and Evans, Formalizing and Estimating Distribution Inference Risks, PETS 2022).

We propose a new attack: the KL Divergence Attack. Using some sample of data, the adversary computes predictions on local models from both distributions as well as the victim’s model. Then, it uses the prediction probabilities to compute KL divergence between the victim’s models and the local models to make its predictions. Our attack outperforms even the current state-of-the-art white-box attacks.

We observe several interesting trends across our experiments. One striking example is that with varying task-property correlation. While intuition suggests increasing inference leakage with increasing correlation between the classifier's task and the property being inferred, we observe no such trend:
Distinguishing accuracy for different task-property pairs for Celeb-A dataset for varying correlation. Task-property correlations are: $\approx 0$ (Mouth Slightly Open-Wavy Hair), $\approx 0.14$ (Smiling-Female), $\approx 0.28$ (Female-Young), and $\approx 0.42$ (Mouth Slightly Open-High Cheekbones).

We evaluate inference risk while relaxing a variety of implicit assumptions of the adversary;s knowledge in black-box setups. Concretely, we evaluate label-only API access settings, different victim-adversary feature extractors, and different victim-adversary model architectures.

RF LR MLP$_2$ MLP$_3$
Random Forest (RF) 12.0 1.7 5.4 4.9
Linear Regression (LR) 13.5 25.9 3.7 5.4
Two-layer perceptron (MLP$_2$) 0.9 0.3 4.2 4.3
Three-layer perceptron (MLP$_3$) 0.2 0.3 4.0 3.8

Consider inference leakage for the Census19 dataset (table above with mean $n_{leaked}$ values) as an example. Inference risk is significantly higher when the adversary uses models with learning capacity similar to the victim, like both using one of (MLP$_2$, MLP$_3$) or (RF, MLP). Interestingly though, we also observe a sharp increase in inference risk when the victim uses models with low capacity, like LR and RF instead of multi-layer perceptrons.

## Defenses

Finally, we evaluate the effectiveness of some empirical defenses, most of which add noise to the training process.

For instance while inference leakage reduces when the victim utilizes DP, most of the drop in effectiveness comes from a mismatch in the victim’s and adversary’s training environments:

Distinguishing accuracy for different for Census19 (Sex). Attack accuracy drops with stronger DP guarantees i.e. decreasing privacy budget $\epsilon$.

Compared to an adversary that does not use DP, there is a clear increase in inference risk (mean $n_{leaked}$ increases to 2.9 for $\epsilon=1.0$, and 4.8 for $\epsilon=0.12$ compared to 4.2 without any DP noise).

Our exploration of potential defenses also reveals a strong connection between model generalization and inference risk (as apparent below, for the case of Celeb-A), suggesting that the defenses that do seem to work are attributable to poor model performance, and not something special about the defense itself (like adversarial training or label noise).
Mean distinguishing accuracy on Celeb-A (Sex), for varying number of training epochs for victim models. Shaded regions correspond to error bars. Distribution inference risk increases as the model trains, and then starts to decrease as the model starts to overfit.

## Summary

The general approach to achieve security and privacy for machine-learning models is to add noise, but our evaluations suggest this approach is not a principled or effective defense against distribution inference. The main reductions in inference accuracy that result from these defenses seem to be due to the way they disrupt the model from learning the distribution well.

Paper: Anshuman Suri, Yifu Lu, Yanjin Chen, David Evans. Dissecting Distribution Inference. In IEEE Conference on Secure and Trustworthy Machine Learning (SaTML), 8-10 February 2023.

## Cray Distinguished Speaker: On Leaky Models and Unintended Inferences

Here’s the slides from my Cray Distinguished Speaker talk on On Leaky Models and Unintended Inferences: [PDF]

The chatGPT limerick version of my talk abstract is much better than mine:

A machine learning model, oh so grand
With data sets that it held in its hand
It performed quite well
But secrets to tell
And an adversary’s tricks it could not withstand.

Thanks to Stephen McCamant and Kangjie Lu for hosting my visit, and everyone at University of Minnesota. Also great to catch up with UVA BSCS alumn, Stephen J. Guy.

The main works I talked about are:

The talk from Randy Pausch (Cray Distinguished Speaker, 1999-2000) that I mentioned is available here: Time Management, November 2007.

The transcript of Seymour Cray’s talk at the University of Virginia is here: An Imaginary Tour of a Biological Computer (Why Computer Professionals and Molecular Biologists Should Start Collaborating). Remarks of Seymour Cray to the Shannon Center for Advanced Studies, University of Virginia. 30 May 1996.

## Attribute Inference attacks are really Imputation

Post by Bargav Jayaraman

Attribute inference attacks have been shown by prior works to pose privacy threat against ML models. However, these works assume the knowledge of the training distribution and we show that in such cases these attacks do no better than a data imputataion attack that does not have access to the model. We explore the attribute inference risks in the cases where the adversary has limited or no prior knowledge of the training distribution and show that our white-box attribute inference attack (that uses neuron activations to infer the unknown sensitive attribute) surpasses imputation in these data constrained cases. This attack uses the training distribution information leaked by the model, and thus poses privacy risk when the distribution is private.

## Prior Attribute Inference Attacks Do Not Pose Privacy Risk

Prior works in attribute inference have mainly considered black-box access to the machine learning model and show successful attribute inference (in terms of attack accuracy) in the case where the adversary has access to the underlying training distribution. Our experiments show that in such cases even an imputation adversary, without access to the model, can achieve high inference accuracy, as shown in the table below:

Census (Race)             Texas-100X (Ethnicity)
Predict Most Common 0.78 0.72
Imputation Attack 0.82 0.72
Yeom et al. Attack 0.65 0.58
Mehnaz et al. Attack 0.06 0.60
WCAI (Our version of Yeom) 0.83 0.74
Comparing accuracy of attribute inference attacks

Thus, these attribute inference attacks seem to not pose any significant privacy risk as the adversary can have similar attack success without access to the model.

## Sensitive Value Inference

Attribute inference risk is inherently asymmetric — identifying a record with minority attribute value (such as Hispanic ethnicity) does not have the same risk as identifying a record with majority attribute value (such as Non-Hispanic ethnicity). Accuracy metric does not capture this. Moreover, attribute inference definition considered by prior works also fails to distinguish these cases. We propose studying a fine-grained version of attribute inference, called sensitive value inference, that considers the attack success in inferring a particular sensitive attribute outcome.

We measure the attack success by evaluating the positive predictive value (PPV) of the inference attack in predicting the top-k candidate records with the sensitive outcome. The PPV values are between 0 and 1, where a higher value denotes a greater attack precision.

## The Neuron Output Attack

Our novel neuron output based white-box attack finds the neurons that are most correlated with the sensitive value. For this attack, the adversary selects records from a hold-out set, sets the unknown target attribute to the sensitive value, and queries the model. The adversary then identifies the set of neurons that have higher activations on average for the records with the sensitive value as the ground-truth. The adversary then uses the aggregate output of these neurons to identify the candidate records with sensitive value.

## Model Leaks Distribution Information

In our experiments, we vary the distribution available to the adversary and also the amount of data from the respective distribution the adversary has to train the inference attack. When the adversary has access to >5000 records from the training distribition (not the same as the training set records), imputataion outperforms all the attribute inference attacks (incuding our white-box neuron output attack). As we decrease the known set size to 500 and 50, the imputation PPV decreases drastically whereas our neuron output attack continues to achieve high PPV. Thus the attack is able to take advantage of the training distribution information leaked by the model. The figure below depicts the case where the adversary has 500 records from the training distribution, and as shown, the neuron output attack surpasses the imputataion.

Neurons correlated to Hispanic ethnicity for a neural network model trained on Texas-100X data set.

We observe similar trend across different distribution settings and across different data sets. Detailed results can be found in the paper.

## Differential Privacy Doesn’t Mitigate the Risk

Prior works have claimed that attribute inference attacks cannot work in the cases where membership inference attacks do not succeed. Hence, some have thought that differential privacy mechanisms which successfully defend against membership inference attacks, also defend against attribute inference attacks. This is based on the attribute advantage metric of Yeom et al. that shows that the difference between the accuracy of inference attack across training and non-training set is bounded by differential privacy. We agree that this is true, as we shown in our experiment results in Table 2 below where the PPV of the neuron output attack is similar across both train and test sets. However, our attribute advantage metric measures the gap between the attack PPV when the adversary has access to the model (i.e., neuron output attack) versus when the adversary does not have model access (i.e., imputataion). As shown in the table below, this is not bounded by differential privacy as the neuron output attack PPV remains more or less the same with or without differential privacy.

Without DP With DP        Train Set Test Set
Imputation Attack 0.62 0.62 0.62 0.63
Neuron Output Attack 0.49 0.49 0.49 0.48
Impact of Differential Privacy (DP) on the PPV of attacks (see table in paper for error margins).
Results show the PPV of attacks in predicting top-100 candidate records.

Since the risk is due to the model leaking distribution information, it is not mitigated by differential privacy noise.

## Conclusion

We show that the attribute inference attacks take advantage of the model leaking sensitive information about the underlying training distribution as opposed to leaking information about individual training records. While this is often considered by researchers to be not a privacy risk since the distribution statistics are supposed to be public knowledge, we argue that when the distribution itself is a private information then any such disclosure poses a severe privacy risk. Existing defenses, such as training the model with differential privacy mechanisms, does not mitigate this distribution privacy risk.

Full paper: Bargav Jayaraman and David Evans. Are Attribute Inference Attacks Just Imputation? (arXiv). In ACM Conference on Computer and Communications Security (CCS 2022).

Talk Video: https://youtu.be/iLy0C5DK2T8

## Congratulations, Dr. Jayaraman!

Congratulations to Bargav Jayaraman for successfully defending his PhD thesis!

Dr. Jayaraman and his PhD committee: Mohammad Mahmoody, Quanquan Gu (UCLA Department of Computer Science, on screen), Yanjun Qi (Committee Chair, on screen), Denis Nekipelov (Department of Economics, on screen), and David Evans

Bargav will join the Meta AI Lab in Menlo Park, CA as a post-doctoral researcher.

Analyzing the Leaky Cauldron: Inference Attacks on Machine Learning

## Balancing Tradeoffs between Fickleness and Obstinacy in NLP Models

Post by Hannah Chen.

Our work on balanced adversarial training looks at how to train models that are robust to two different types of adversarial examples:

Hannah Chen, Yangfeng Ji, David Evans. Balanced Adversarial Training: Balancing Tradeoffs between Fickleness and Obstinacy in NLP Models. In The 2022 Conference on Empirical Methods in Natural Language Processing (EMNLP), Abu Dhabi, 7-11 December 2022. [ArXiv]

At the broadest level, an adversarial example is an input crafted intentionally to confuse a model. However, most work focus on the defintion as an input constructed by applying a small perturbation that preserves the ground truth label but changes model’s output (Goodfellow et al., 2015). We refer it as a fickle adversarial example. On the other hand, attackers can target an opposite objective where the inputs are made with minimal changes that change the ground truth labels but retain model’s predictions (Jacobsen et al., 2018). We refer these malicious inputs as obstinate adversarial examples.

Adversarial examples for texts (Red: synonym substitution, Blue: antonym substitution)

## Distance-Oracle Misalignment

Previous work from (Tramer et al., 2020) show that for image classification models, increasing robustness against fickle adversarial examples may also increase vulnerability to obstinate adversarial attacks. They suggested the reason behind this is may be the distance-oracle misalignment during fickle adversarial training. The norm bounded perturbation used for certified robust training may not align with the ground truth decision boundary. We hypothesize that this phenomenon may also exist in NLP models since the automatically-generated adversarial examples for NLP models can be imperfect sometimes, e.g., synonym word substitutions for constructing fickle adversarial examples may not preserve the ground truth label of the input.

To test our hypothsis, we perform obstinate adversarial attacks on models trained with normal training and fickle adversarial training. We use antonym word substitution for obstinate attack and SAFER (Ye et al., 2020), a certified robust training for NLP models, as the fickle adversarial defense. We visualize the antonym attack success rate on models trained with SAFER at each training epoch. We found that as the synonym attack success rate decreases over the course of training, the antonym attack success rate increases as well. The antonym attack success rate is also higher than the normal training baseline. This results prove our hypothesis that optimizing only fickle adversarial robustness can result in models being more vulnerable to obstinate adversarial examples.

We adapt constrastive learning by pairing fickle adversarial examples with the original examples as positive pairs and obstinate adversarial examples with the original examples as negative pairs. The goal of training is to minimize the distance between the postive pairs and maximize the distance between the negative pairs. We propose BAT-Pairwise and BAT-Triplet, where each combines a normal training objective with a pairwise or triplet loss.

We evaluate BAT based on synonym (fickle) and antonym (obstinate) attack success rate and compare it with normal training, and two fickle adversarial defenses, A2T (vanilla adversarial training) (Yoo and Qi, 2021) and SAFER (certified robust training). We show that both BAT-Pairwise and BAT-Triple result in better robustness against antonym attacks compared to other training baselines and are more robust against synonym attacks than the normal training method. While fickle adversarial defenses (A2T and SAFER) perform best when evaluated solely based on fickleness robustness, they have worse obstinacy robustness. Our proposed method gives a better balance between the two types of robustness.

We compare the learned representations of models trained with BAT and other training baselines. We project the embeddings to 2 dimensional space with t-SNE. We see that boh fickle and obstinate examples are close to the original examples when the model is trained with normal training or SAFER. With BAT-Pairwise and BAT-Triplet, only the fickle examples and the original examples are close to each other while the obstinate examples are further away from them. This results match with BAT’s training goal and show that BAT can mitigate the distance-oracle misalignment.

## Summary

We show that robustness tradeoffs between ficklenss and obstinacy exist in NLP models. To counter this, we propose Balanced Adversarial Training (BAT) and show that it helps increase robustness against both fickle and obstinate adversarial examples.

Paper: [ArXiv]