Obfuscated Command Line Detection Using Machine Learning

This blog post presents a machine learning (ML) approach to solving
an emerging security problem: detecting obfuscated Windows command
line invocations on endpoints. We start out with an introduction to
this relatively new threat capability, and then discuss how such
problems have traditionally been handled. We then describe a machine
learning approach to solving this problem and point out how ML vastly
simplifies development and maintenance of a robust obfuscation
detector. Finally, we present the results obtained using two different
ML techniques and compare the benefits of each.

Introduction

Malicious actors are increasingly “living off the land,” using
built-in utilities such as PowerShell and the Windows Command
Processor (cmd.exe) as part of their infection workflow in an effort
to minimize
the chance of detection and bypass whitelisting
defense
strategies. The release of new obfuscation tools makes detection of
these threats even more difficult by adding a layer of indirection
between the visible syntax and the final behavior of the command. For
example, Invoke-Obfuscation
and Invoke-DOSfuscation
are two recently released tools that automate
the obfuscation of Powershell and Windows command lines
respectively.

The traditional pattern matching and rule-based approaches for
detecting obfuscation are difficult to develop and generalize, and can
pose a huge maintenance headache for defenders. We will show how using
ML techniques can address this problem.

Detecting obfuscated command lines is a very useful technique
because it allows defenders to reduce the data they must review by
providing a strong filter for possibly malicious activity. While there
are some examples of “legitimate” obfuscation in the wild, in the
overwhelming majority of cases, the presence of obfuscation generally
serves as a signal for malicious intent.

Background

There has been a long history of obfuscation being employed to hide
the presence of malware
, ranging from encryption of malicious
payloads (starting with the Cascade
virus
) and obfuscation of strings, to JavaScript
obfuscation
. The purpose of obfuscation is two-fold:

  • Make it harder to find
    patterns in executable code, strings or scripts that can easily be
    detected by defensive software.
  • Make it harder for reverse
    engineers and analysts to decipher and fully understand what the
    malware is doing.

In that sense, command line obfuscation is not a new problem – it is
just that the target
of obfuscation
(the Windows Command Processor) is relatively
new. The recent release of tools such as Invoke-Obfuscation (for
PowerShell) and Invoke-DOSfuscation (for cmd.exe) have demonstrated
just how flexible these commands are, and how even incredibly complex
obfuscation will still run commands effectively.

There are two categorical axes in the space of obfuscated vs.
non-obfuscated command lines: simple/complex and clear/obfuscated (see
Figure 1 and Figure 2). For this discussion “simple” means generally
short and relatively uncomplicated, but can still contain obfuscation,
while “complex” means long, complicated strings that may or may not be
obfuscated. Thus, the simple/complex axis is orthogonal to
obfuscated/unobfuscated. The interplay of these two axes produce many
boundary cases where simple heuristics to detect if a script is
obfuscated (e.g. length of a command) will produce false positives on
unobfuscated samples. The flexibility of the command line processor
makes classification a difficult task from an ML perspective.

Figure 1: Dimensions of obfuscation

Figure 2: Examples of weak and strong obfuscation

Traditional Obfuscation Detection

Traditional obfuscation detection can be split into three
approaches. One approach is to write a large number of complex regular
expressions to match the most commonly abused syntax of the Windows
command line. Figure 3 shows one such regular expression that attempts
to match ampersand chaining with a call command, a common pattern seen
in obfuscation. Figure 4 shows an example command sequence this regex
is designed to detect.

Figure 3: A common obfuscation pattern
captured as a regular expression

Figure 4: A common obfuscation pattern
(calling echo in obfuscated fashion in this example)

There are two problems with this approach. First, it is virtually
impossible to develop regular expressions to cover every possible
abuse of the command line. The flexibility of the command line results
in a non-regular
language
, which is feasible yet impractical to express using
regular expressions. A second issue with this approach is that even if
a regular expression exists for the technique a malicious sample is
using, a determined attacker can make minor modifications to avoid the
regular expression. Figure 5 shows a minor modification to the
sequence in Figure 4, which avoids the regex detection.

Figure 5: A minor change (extra carets)
to an obfuscated command line that breaks the regular expression in
Figure 3

The second approach, which is closer to an ML approach, involves
writing complex if-then rules. However, these rules are hard to
derive, are complex to verify, and pose a significant maintenance
burden as authors evolve to escape detection by such rules. Figure 6
shows one such if-then rule.

Figure 6: An if-then rule that *may*
indicate obfuscation (notice how loose this rule is, and how false
positives are likely)

A third approach is to combine regular expressions and if-then
rules. This greatly complicates the development and maintenance
burden, and still suffers from the same weaknesses that make the first
two approaches fragile. Figure 7 shows an example of an if-then rule
with regular expressions. Clearly, it is easy to appreciate how
burdensome it is to generate, test, maintain and determine the
efficacy of such rules.

Figure 7: A combination of an if-then
rule with regular expressions to detect obfuscation (a real
hand-built obfuscation detector would consist of tens or hundreds of
rules and still have gaps in its detection)

The ML Approach – Moving Beyond Pattern Matching and Rules

Using ML simplifies the solution to these problems. We will
illustrate two ML approaches: a feature-based approach and a
feature-less end-to-end approach.

There are some ML techniques that can work with any kind of raw data
(provided it is numeric), and neural networks are a prime example.
Most other ML algorithms require the modeler to extract pertinent
information, called features, from raw data before they are fed
into the algorithm. Some examples of this latter type are tree-based
algorithms, which we will also look at in this blog (we described the
structure
and uses of Tree-Based algorithms
in a previous blog post, where
we used a Gradient-Boosted Tree-Based Model).

ML Basics – Neural Networks

Neural networks are a type of ML algorithm that have recently become
very popular and consist of a series of elements called
neurons. A neuron is essentially an element that takes a set of
inputs, computes a weighted sum of these inputs, and then feeds the
sum into a non-linear function. It has been shown that a relatively
shallow network of neurons can approximate any continuous mapping
between input and output. The specific type of neural network we used
for this research is what is called a Convolutional Neural Network
(CNN), which was developed primarily for computer vision applications,
but has also found success in other domains including natural language
processing. One of the main benefits of a neural network is that it
can be trained without having to manually engineer features.

Featureless ML

While neural networks can be used with feature data, one of the
attractions of this approach is that it can work
with raw data
(converted into numeric form) without doing any
feature design or extraction. The first step in the model is
converting text data into numeric form. We used a character-based
encoding where each character type was encoded by a real valued
number. The value was automatically derived during training and
conveys semantic information about the relationships between
characters as they apply to cmd.exe syntax.

Feature-Based ML

We also experimented with hand-engineered features and a Gradient
Boosted Decision Tree algorithm. The features developed for this model
were largely statistical in nature – derived from the presence and
frequency of character sets and keywords. For example, the presence of
dozens of ‘%’ characters or long, contiguous strings might contribute
to detecting potential obfuscation. While any single feature will not
perfectly separate the two classes, a combination of features as
present in a tree-based model can learn flexible patterns in the data.
The expectation is that those patterns are robust and can generalize
to future obfuscation variants.

Data and Experiments

To develop our models, we collected non-obfuscated data from tens of
thousands of endpoint events and generated obfuscated data using a
variety of methods in Invoke-DOSfuscation. We developed our models
using roughly 80 percent of the data as training data, and tested them
on the remaining 20 percent. We ensured that our train-test split was
stratified. For featureless ML (i.e. neural networks), we simply input
Unicode code points into the first layer of the CNN model. The first
layer converts the code point into semantically meaningful numerical
representations (called embeddings) before feeding it into the rest of
the neural network.

For the Gradient Boosted Tree method, we generated a number of
features from the raw command lines. The following are some of them:

  • Length of the command
    line
  • The number of carets in the command line
  • The
    count of pipe symbols
  • The fraction of white space in the
    command line
  • The fraction of special characters
  • Entropy of the string
  • The frequency of the strings
    “cmd” and “power” in the command line

While each of these features individually is a weak signal and could
not possibly be a good discriminator on its own, a flexible classifier
such as a Gradient Boosted Tree – trained on sufficient data with
these features – is able to classify obfuscated and non-obfuscated
command lines in spite of the aforementioned difficulties.

Results

Evaluated against our test set, we were able to get nearly identical
results from our Gradient Boosted Tree and neural network models.

The results for the GBT model were near perfect with metrics such as
F1-score, precision, and recall all being close to 1.0. The CNN model
was slightly less accurate.

While we certainly do not expect perfect results in a real-world
scenario, these lab results were nonetheless encouraging. Recall that
all of our obfuscated examples were generated by one source, namely
the Invoke-DOSfuscation tool. While Invoke-DOSfuscation generates a
wide variety of obfuscated samples, in the real world we expect to see
at least some samples that are quite dissimilar from any that
Invoke-DOSfuscation generates. We are currently collecting real world
obfuscated command lines to get a more accurate picture of the
generalizability of this model on obfuscated samples from actual
malicious actors. We expect that command obfuscation, similar to
PowerShell obfuscation before it, will continue to emerge in new
malware families.

As an additional test we asked Daniel Bohannon (author of
Invoke-DOSfuscation, the Windows command line obfuscation tool) to
come up with obfuscated samples that in his experience would be
difficult for a traditional obfuscation detector. In every case, our
ML detector was still able to detect obfuscation. Some examples are
shown in Figure 8.

Figure 8: Some examples of obfuscated
text used to test and attempt to defeat the ML obfuscation detector
(all were correctly identified as obfuscated text)

We also created very cryptic looking texts that, although valid
Windows command lines and non-obfuscated, appear slightly obfuscated
to a human observer. This was done to test efficacy of the detector
with boundary examples. The detector was correctly able to classify
the text as non-obfuscated in this case as well. Figure 9 shows one
such example.

Figure 9: An example that appears on
first glance to be obfuscated, but isn’t really and would likely
fool a non-ML solution (however, the ML obfuscation detector
currently identifies it as non-obfuscated)

Finally, Figure 10 shows a complicated yet non-obfuscated command
line that is correctly classified by our obfuscation detector, but
would likely fool a non-ML detector based on statistical features (for
example a rule-based detector with a hand-crafted weighing scheme and
a threshold, using features such as the proportion of special
characters, length of the command line or entropy of the command line).

Figure 10: An example that would likely
be misclassified by an ML detector that uses simplistic statistical
features; however, our ML obfuscation detector currently identifies
it as non-obfuscated

CNN vs. GBT Results

We compared the results of a heavily tuned GBT classifier built
using carefully selected features to those of a CNN trained with raw
data (featureless ML). While the CNN architecture was not heavily
tuned, it is interesting to note that with samples such as those in
Figure 10, the GBT classifier confidently predicted non-obfuscated
with a score of 19.7 percent (the complement of the measure of the
classifier’s confidence in non-obfuscation). Meanwhile, the CNN
classifier predicted non-obfuscated with a confidence probability of
50 percent – right at the boundary between obfuscated and
non-obfuscated. The number of misclassifications of the CNN model was
also more than that of the Gradient Boosted Tree model. Both of these
are most likely the result of inadequate tuning of the CNN, and not a
fundamental shortcoming of the featureless approach.

Conclusion

In this blog post we described an ML approach to detecting
obfuscated Windows command lines, which can be used as a signal to
help identify malicious command line usage. Using ML techniques, we
demonstrated a highly accurate mechanism for detecting such command
lines without resorting to the often inadequate and costly technique
of maintaining complex if-then rules and regular expressions. The more
comprehensive ML approach is flexible enough to catch new variations
in obfuscation, and when gaps are detected, it can usually be handled
by adding some well-chosen evader samples to the training set and
retraining the model.

This successful application of ML is yet another demonstration of
the usefulness of ML in replacing complex manual or programmatic
approaches to problems in computer security. In the years to come, we
anticipate ML to take an increasingly important role both at FireEye
and in the rest of the cyber security industry.

*** This is a Security Bloggers Network syndicated blog from Threat Research authored by Threat Research Blog. Read the original post at: http://www.fireeye.com/blog/threat-research/2018/11/obfuscated-command-line-detection-using-machine-learning.html