June 24, 2021

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Training Transformers for Cyber Security Tasks: A Case Study on
Malicious URL Prediction

Training Transformers for Cyber Security Tasks: A Case Study on Malicious URL Prediction

Highlights       

  • Perform a case study on using Transformer models to solve
    cyber security problems
  • Train a Transformer model to detect
    malicious URLs under multiple training regimes
  • Compare our
    model against other deep learning methods, and show it performs
    on-par with other top-scoring models
  • Identify issues with
    applying generative pre-training to malicious URL detection, which
    is a cornerstone of Transformer training in natural language
    processing (NLP) tasks
  • Introduce novel loss function that
    balances classification and generative loss to achieve improved
    performance on the malicious URL detection task

Introduction

Over the past three years Transformer machine learning (ML) models,
or “Transformers” for short, have yielded impressive breakthroughs in
a variety of sequence modeling problems, specifically natural language
processing (NLP). For example, OpenAI’s latest GPT-3
model is capable of generating long segments of grammatically-correct
prose from scratch. Spinoff models, such as those developed for
question and answering, are capable of correlating context over
multiple sentences. AI Dungeon, a
single and multiplayer text adventure game, uses Transformers to
generate plausible unlimited content in a variety of fantasy settings.
Transformers’ NLP modeling capabilities are apparently so powerful
that they pose security risks in their own right, in terms of their potential
power to spread disinformation
, yet on the other side of the
coin, they can be used as powerful tools to detect and mitigate
disinformation campaigns. For example, in previous
research
by the FireEye Data Science team, a NLP Transformer was
fine-tuned to detect disinformation on social media sites.

Given the power of these Transformer models, it seems natural to
wonder if we can apply them to other types of cyber security problems
that do not necessarily involve natural language, per se. In this blog
post, we discuss a case study in which we apply Transformers to
malicious URL detection. Studying Transformer performance on URL
detection problem is a first logical step to extending Transformers to
more generic cyber security tasks, since URLs are not technically
natural language sequences but share some common characteristics with NLP.

In the following sections, we outline a typical Transformer
architecture and discuss how we adapt it to URLs with a
character-focused tokenization. We then discuss loss functions we
employ to guide the training of the model, and finally compare our
training approaches to more conventional ML-based modeling options.

Adapting Transformers to URLs

Our URL Transformer operates at the character level, where each
character in the URL corresponds to an input token. When a URL is
input to our Transformer, it is appended with special tokens—a
classification token (“CLS”) that conditions the model to produce a
prediction and padding tokens (“PAD”) that normalize the input to a
fixed length to allow for parallel training. Each token in the input
string is then projected into a character embedding space, followed by
a stack of Attention and Feed-Forward
Neural Network (FFNN)
layers. This stack of layers is similar to
the architecture introduced in the original
Transformers paper
. At a high level, the Attention layers allow
each input to be associated with long-distance context of other
characters that are important for the classification task, similar to
the notion of attention in humans, while the FFNN layers provide
capacity for learning the relationships among the combination of
inputs and their respective contexts. An illustration of our
architecture is shown in Figure 1.

Additionally, the URL Transformer employs a masking strategy in its
Attention calculation, which enforces a left-to-right (L-R)
dependence. This means that only input characters from the left of a
given character influence that character’s representation in each
layer of the attention stack. The network outputs one embedding for
each input character, which captures all information learned by the
model about the character sequence up to that point in the input.

Once the model is trained, we can use the URL Transformer to perform
several different tasks, such as generatively predicting the next
character in the input sequence by using the sequence embedding () as
an input to another neural network with as softmax output over the
possible vocabulary of characters. A specific example of this is shown
in Figure 1, where we take the embedding of the input “firee”() and
use it to predict the next most likely character, “y.” Similarly, we
can use the embedding produced after the classification token to
predict other properties of the input sequences, such as their
likelihood of maliciousness.

Training Transformers for Cyber Security Tasks: A Case Study on
Malicious URL Prediction

Figure 1: High-level overview of the URL
Transformer architecture

Loss Functions and Training Regimes

With the model architecture in hand, we now turn to the question of
how we train the model to most effectively detect malicious URLs. Of
course, we can train this model in a similar way to other supervised
deep learning classifiers by: (1) making predictions on samples from a
labeled training set, (2) using a loss function
to measure the quality of our predictions, and (3) tune model
parameters (i.e., weights) via backpropagation.
However, the nature of the Transformer model allows for several
interesting variations to this training regime. In fact, one of
the reasons that Transformers have become so popular for NLP tasks is
because they allow for self-supervised
generative pre-training, which takes advantage of massive amounts of
unlabeled data to help the model learn general characteristics of the
input language before being fine-tuned on the ultimate task at-hand
(e.g., question answering, sentiment analysis, etc.). Here, we outline
some of the training regimes we explored for our URL Transformer model.

Direct Label Prediction (Decode-To-Label)

Using a training set of URLs with malicious and benign labels, we
can treat the URL Transformer architecture as a feature extractor,
whose outputs we use as the input to a traditional classifier (e.g.,
FFNN or even a random forest). When using a FFNN as our classifier, we
can backpropagate the classification loss (e.g., binary cross-entropy)
through both the classifier and the Transformer network to adjust the
weights to perform classification. This training regime is the
baseline for our experiments and is how most deep learning models are
trained for classification tasks.

Next-Character Prediction Pre-Training and Fine-Tuning

Beyond the baseline classification training regime, the NLP
literature suggests that one can learn a self-supervised embedding of
the input sequence by training the Transformer to perform a
next-character prediction task, then fine-tuning the learned
representation for the classification problem. A key advantage of this
approach is that data used for pre-training does not require malicious
or benign labels; instead, the next characters in a URL serve as the
labels to be predicted from prior characters in the sequence. This is
similar to the example given in Figure 1, where the embedding output
is used to predict the next character, “y,” in “fireeye.com.” Overall,
this training regime allows us to take advantage of the massive amount
of unlabeled data that is typically available in cyber
security-related problems.

The overall structure of the architecture for this regime is similar
to the aforementioned binary classification task, with FFNN layers
added for classification. However, since we are now predicting
multiple classes (i.e., one class per input character in the
vocabulary), we must apply a softmax function to the output to induce
a probability distribution over the potential output characters. Once
the Transformer portion of the network is pre-trained in this way, we
can swap the FFNN classification layers focused on character
prediction with new layers that will be trained for the malicious URL
classification problem, as in the decode-to-label case.

Balanced Mixed-Objective Training

Prior
work
has shown that imbuing the training process with additional
knowledge outside of the primary task can help constrain the learning
process, and ultimately result in better models. For instance, a
malware classifier might train using loss functions that capture
malicious/benign classification, malware family prediction, and tag
prediction tasks as a mechanism to provide the classifier with broader
understanding of the problem than looking at malicious/benign labels
in isolation.

Inspired by these findings, we also introduced a mixed-objective
training regime for our URL Transformer, where we train for binary
classification and next-character prediction simultaneously. At each
iteration of training, we compute a loss multiplier such that each
loss contribution is fixed prior to backpropagation. This ensures that
neither loss term dominates during training. Specifically, for
minibatch i, let the net loss LMixed be computed as follows:

 

Given hyperparameters a and b, defined such that a + b: = 1, we compute constant a so that the net loss contribution of
LCLS
to LMixed is a and the net contribution of LNext
to LMixed is b. For our evaluations, we set a := b := 0.5, effectively requiring that
the model equally balance its ability to generate the next character
and accurately predict malicious URLs.

Evaluation

To evaluate our URL Transformer model and better understand the
impact of the three training regimes discussed earlier, we collected a
training dataset of over 1M labeled malicious and benign URLs, which
was split into roughly 700K training samples, 100K validation samples,
and 200k test samples. Additionally, we also developed an unlabeled
pre-training dataset of 20M URLs.

Using this data, we performed four different training runs for our
Transformer model:

  1. DecodeToLabel (Baseline): Using strictly the binary
    cross-entropy loss on the embedded classification features over the
    entire sequence, we trained the model for 15 epochs using the
    training set.
  2. MixedObjective: We trained the model for 15 epochs on the
    training set, using both the embedded classification features and
    the embedded next-character prediction features.
  3. FineTune: We pre-trained the model for 15 epochs on the
    next-character prediction task using the training set, ignoring the
    malicious/benign labels. We then froze weights over the first 16
    layers of the model and trained the model for an additional 15
    epochs using a binary cross-entropy loss on the classification
    labels.
  4. FineTune 20M: We performed pre-training on the next-character
    prediction task using the 20M URL dataset, pre-training for 2
    epochs. We then froze weights over the first 16 layers of the
    Transformer and trained for 15 epochs on the binary classification
    task.

The ROC curve shown in Figure 2 compares the performance of these
four training regimes. Here, our baseline DecodeToLabel model (red)
yielded a ROC curve with 0.9484 AUC, while the MixedObjective model
(green) slightly outperformed the baseline with an AUC of 0.956.
Interestingly, both of the fine-tuning models yielded poor
classification results, which is counter to the established practice
of these Transformer models in the NLP domain.



Figure 2: ROC curves for four URL
Transformer training regimes

To assess the relative efficacy of our Transformer models on this
dataset, we also fit several other types of benchmark models developed
for URL classification: (1) a Random Forest model on SME-derived
features, (2) a 1D Convolutional Neural Network (CNN) model on
character embeddings, and (3) a Long Short-Term Memory (LSTM) neural
network on character embeddings. Details of these models can be found
in our white paper,
however we find that our top performing Transformer model performs
on-par with the best performing non-Transformer baseline (a 1D CNN
model), which perhaps indicates that the long-range dependencies
typically learned by Transformer models are not as useful in the case
of malicious URL detection.



Figure 3: ROC curves comparing URL
Transformer to other benchmark URL classification models

Summary

Our experiments suggest that Transformers can achieve performance
comparable to or better than that of other top-performing models for
URL classification, though the details of how to achieve that
performance differ from common practice. Contrary to findings
from the NLP domain
, wherein self-supervised pre-training
substantially enhances performance in a fine-tuned classification
task, similar pretraining approaches actually diminish performance for
malicious URL detection. This suggests that the next character
prediction task has too little apparent correlation with the task of
malicious/benign prediction for effective/stable transfer.

Interestingly, utilizing next-character prediction as an auxiliary
loss function in conjunction with a malicious/benign loss yields
improvements over training solely to predict the label. We hypothesize
that while pre-training leads to a relatively poor generative model
due to randomized content in the URLs within our dataset, a
malicious/benign loss may serve to better condition the generative
model learned by the next-character prediction task, distilling a
subset of relevant information. It may also be the case that the
long-distance relationships that are key to the generative
pre-training task are not as important for the final malicious URL
classification, as evidenced by the performance of the 1D CNN model.

Note that we did not perform a rigorous hyperparameter search for
our Transformer, since this research was primarily concerned with loss
functions and training regimes. Therefore, it is still an open
question as to whether a more optimal architecture, specifically
designed for this classification task, could substantially outperform
the models described here.

While our URL dataset is not representative of all data in the cyber
security space, the difficulty of obtaining a readily fine-tuned model
from self-supervised pre-training suggests that this approach is
unlikely to work well for training Transformers on longer sequences or
sequences with lesser resemblance to natural language (e.g., PE
files), but an auxiliary loss might work.

Details about this research and additional results can be found in
our associated white paper.