your face has been hijacked
Machine learning with deep neural networks (commonly dubbed "deep learning") has taken the world by storm, smashing record after record in a wide variety of difficult tasks from fields that were largely unexplored in previous years, such as computer vision, speech recognition and natural language processing. One computer vision task that benefits from such clear gains is facial recognition (identifying a person based on his/her face)---a deep neural network being the primary tool used for this purpose at Facebook, among other places.
One natural extension of the above could be to exploit neural networks within a secure application, in order to authenticate a person based on a shot of their face. Unfortunately, despite the apparently superb performance of such models, it is fairly easy to construct inputs which can trick the network into authenticating a stranger (commonly known as adversarial examples). We built FaceJack in order to illuminate this concept. In particular, we'd like to emphasise:
- how simple it is to generate such "fooling" inputs algorithmically, if one has access to the neural network used for facial recognition (either directly or through an API).
- how imperceptibly close the "fooling" inputs can be to legitimately generated inputs;
- how this attack may be executed in real-time, requiring only a mid-range GPU.
But let's take it slowly---what even are adversarial inputs?
Adversarial inputs are made possible by the very design of neural networks. On a high level, a neural network consumes an input, performs several transformations to it, in order to predict a corresponding output. The network parameters are adjusted by running the network on a training set (a set of known input/output pairs from which the network needs to generalise). The network's transformations are designed to be differentiable, so that the network can be efficiently trained by:
- Feeding an input to the network, computing a prediction
- Computing an error of the prediction with respect to the expected output
- Propagating the error backwards through the network, updating parameters as we go.
The network's differentiability allows us to consider the error function in its parameters, for a fixed input and output---so we can optimise them. However, it also allows us to set up an error function in the input, for a fixed choice of parameters---so we can modify the input to produce a desirable output. If the "desirable" output classification is one that the original input does not belong to (e.g. classifying my face as John Travolta), then the constructed input represents an adversarial example. Deep neural networks are particularly vulnerable to such inputs, for three main reasons:
- Computing an adversarial example usually only requires a crude approximation of the gradient of the desired output with respect to the input image---often, only the sign of this gradient for each input pixel is sufficient.
- The computed adversarial examples are often imperceptibly similar to the original input---in fact, there is an entire space of adversarial inputs surrounding any correctly classified image, as Szegedy et al. have demonstrated in 2013.
- Even worse---what's adversarial for one network architecture will very often be adversarial for a completely different network as well---as they are often trained on the same datasets!
Therefore, using neural networks in secure applications requires particular care, as adversarial inputs give rise to a potentially unforeseen covert channel for an exploit.
What have we done?
We have built FaceJack as a simple representative of such an exploit:
- We have fine-tuned a deep convolutional neural network (CNN) based on the VGG-face architecture, in order to authenticate one of our team members (Laurynas) as an administrator of a secure system;
- The authentication system leverages a laptop web cam---detected faces in the camera's view are submitted to the network for classification;
- We have planted a "hack switch", capable of intercepting the input and performing adversarial training on it before submitting it for classification---this resulted in a 100% success rate for authenticating as Laurynas, regardless of your facial features.
Hopefully, FaceJack has achieved its objective of highlighting this important issue in a clear and concise fashion. We hope to expand it in the near future with further authentication attacks, for example speech recognition-based ones.
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