July 31, 2021

Malware Protection

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capa 2.0: Better, Faster, Stronger

capa 2.0: Better, Faster, Stronger

We are excited to announce version 2.0 of our open-source tool called
capa. capa automatically identifies capabilities in programs using an
extensible rule set. The tool supports both malware triage and deep
dive reverse engineering. If you haven’t heard of capa before, or need
a refresher, check out our first
blog post
. You can download capa 2.0 standalone binaries from
the project’s release page and
checkout the source code on GitHub.

capa 2.0 enables anyone to contribute rules more easily, which makes
the existing ecosystem even more vibrant. This blog post details the
following major improvements included in capa 2.0:

  • New features and enhancements for the capa
    IDA Pro plugin, allowing you to interactively explore
    capabilities and write new rules without switching windows
  • More concise and relevant results via identification of library
    functions using FLIRT and the release of accompanying open-source
    FLIRT signatures
  • Hundreds of new rules describing
    additional malware capabilities, bringing the collection up to 579
    total rules, with more than half associated with ATT&CK
  • Migration to Python 3, to make it easier to
    integrate capa with other projects

capa explorer and Rule Generator

capa explorer is an IDAPython plugin that shows capa results
directly within IDA Pro. The version 2.0 release includes many
additions and improvements to the plugin, but we’d like to highlight
the most exciting addition: capa explorer now helps you write new capa
rules directly in IDA Pro!

Since we spend most of our time in reverse engineering tools such as
IDA Pro analyzing malware, we decided to add a capa rule generator.
Figure 1 shows the rule generator interface.

capa 2.0: Better, Faster, Stronger

Figure 1: capa explorer rule generator interface

Once you’ve installed capa explorer using the Getting
guide, open the plugin by navigating to Edit >
Plugins > FLARE capa explorer. You can start using
the rule generator by selecting the Rule Generator tab at the
top of the capa explorer pane. From here, navigate your IDA Pro
Disassembly view to the function containing a technique you’d like to
capture and click the Analyze button. The rule generator will
parse, format, and display all the capa features that it finds in your
function. You can write your rule using the rule generator’s three
main panes: Features, Preview, and Editor. Your
first step is to add features from the Features pane.

The Features pane is a tree view containing all the capa
features extracted from your function. You can filter for specific
features using the search bar at the top of the pane. Then, you can
add features by double-clicking them. Figure 2 shows this in action.

Figure 2: capa explorer feature selection

As you add features from the Features pane, the rule
generator automatically formats and adds them to the Preview
and Editor panes. The Preview and Editor panes
help you finesse the features that you’ve added and allow you to
modify other information like the rule’s metadata.

The Editor pane is an interactive tree view that displays the
statement and feature hierarchy that forms your rule. You can reorder
nodes using drag-and-drop and edit nodes via right-click context
menus. To help humans understand the rule logic, you can add
descriptions and comments to features by typing in the
Description and Comment columns. The rule generator
automatically formats any changes that you make in the Editor
pane and adds them to the Preview pane. Figure 3 shows how to
manipulate a rule using the Editor pane.

Figure 3: capa explorer editor pane

The Preview pane is an editable textbox containing the final
rule text. You can edit any of the text displayed. The rule generator
automatically formats any changes that you make in the Preview
pane and adds them to the Editor pane. Figure 4 shows how to
edit a rule directly in the Preview pane.

Figure 4: capa explorer preview pane

As you make edits the rule generator lints your rule and notifies
you of any errors using messages displayed underneath the Preview
pane. Once you’ve finished writing your rule you can save it to
your capa rules directory by clicking the Save button. The rule
generator saves exactly what is displayed in the Preview pane.
It’s that simple!

We’ve found that using the capa explorer rule generator
significantly reduces the amount of time spent writing new capa rules.
This tool not only automates most of the rule writing process but also
eliminates the need to context switch between IDA Pro and your
favorite text editor allowing you to codify your malware knowledge
while it’s fresh in your mind.

To learn more about capa explorer and the rule generator check out

Library Function Identification Using FLIRT

As we wrote hundreds of capa rules and inspected thousands of capa
results, we recognized that the tool sometimes shows distracting
results due to embedded library code. We believe that capa needs to
focus its attention on the programmer’s logic and ignore supporting
library code. For example, highly optimized C/C++ runtime routines and
open-source library code enable a programmer to quickly build a
product but are not the product itself. Therefore, capa results
should reflect the programmer’s intent for the program rather than a
categorization of every byte in the program.

Compare the capa v1.6 results in Figure 5 versus capa v2.0 results
in Figure 6. capa v2.0 identifies and skips almost 200 library
functions and produces more relevant results.

Figure 5: capa v1.6 results without
library code recognition

Figure 6: capa v2.0 results ignoring
library code functions

So, we searched for a way to differentiate a programmer’s code from
library code.

After experimenting with a few strategies, we landed upon the Fast
Library Identification and Recognition Technology (FLIRT) developed
by Hex-Rays
. Notably, this technique has remained stable and
effective since 1996, is fast, requires very limited code analysis,
and enjoys a wide community in the IDA Pro userbase. We figured out
how IDA Pro matches FLIRT signatures and re-implemented a
matching engine in Rust
with Python bindings.
Then, we built an open-source signature
that covers many of the library routines encountered in modern
malware. Finally, we updated capa to use the new signatures to guide
its analysis.

capa uses these signatures to differentiate library code from a
programmer’s code. While capa can extract and match against the names
of embedded library functions, it will skip finding capabilities and
behaviors within the library code. This way, capa results better
reflect the logic written by a programmer.

Furthermore, library function identification drastically improves
capa runtime performance: since capa skips processing of library
functions, it can avoid the costly rule matching steps across a
substantial percentage of real-world functions. Across our testbed of
206 samples, 28% of the 186,000 total functions are recognized as
library code by our function signatures. As our implementation can
recognize around 100,000 functions/sec, library function
identification overhead is negligible and capa is approximately 25%
faster than in 2020!

Finally, we introduced a new feature class that rule authors can use
to match recognized library functions: function-name. This feature matches at the
file-level scope. We’ve already started using this new capability to
recognize specific implementations of cryptography routines, such as
AES provided by Crypto++, as
shown in the example rule in Figure 7.

Figure 7: Example rule using
function-name to recognize AES via Crypto++

As we developed rules for interesting behaviors, we learned a lot
about where uncommon techniques are used legitimately. For example, as
malware analysts, we most commonly see the cpuid instruction alongside anti-analysis checks,
such as in VM detection routines. Therefore, we naively crafted rules
to flag this instruction. But, when we tested it against our testbed,
the rule matched most modern programs because this instruction is
often legitimately used in high-optimized routines, such as memcpy, to opt-in to newer CPU features. In
hindsight, this is obvious, but at the time it was a little surprising
to see cpuid in around 15% of all
executables. With the new FLIRT support, capa recognizes the optimized
memcpy routine embedded by Visual Studio
and won’t flag the embedded cpuid
instruction, as its not part of the programmer’s code.

When a user upgrades to capa 2.0, they’ll see that the tool runs
faster and provides more precise results.

Signature Generation

To provide the benefits of python-flirt to all
users (especially those without an IDA Pro license) we have spent
significant time to create a comprehensive FLIRT signature set for the
common malware analysis use-case. The signatures come included with
capa and are also available at our GitHub under the
Apache 2.0 license. We believe that other projects can benefit greatly
from this. For example, we expect the performance of FLOSS to improve
once we’ve incorporated library function identification. Moreover, you
can use our signatures with IDA Pro to recognize more library code.

Our initial signatures include:

  • From Microsoft Visual Studio (VS), for all major versions from
    VS6 to VS2019:

    • C and C++ run-time libraries
    • Active
      Template Library (ATL) and Microsoft Foundation Class (MFC)
  • The following open-source projects
    as compiled with VS2015, VS2017, and VS2019:

    • CryptoPP
    • curl
    • Microsoft Detours
    • Mbed TLS (previously
    • OpenSSL
    • zlib

Identifying and collecting the relevant library and object files
took a lot of work. For the older VS versions this was done manually.
For newer VS versions and the respective open-source projects we were
able to automate the process using vcpgk and Docker.

We then used the IDA Pro FLAIR utilities to convert gigabytes of
executable code into pattern files and then into signatures. This
process required extensive research and much trial and error. For
instance, we spent two weeks testing and exploring the various FLAIR
options to understand the best combination. We appreciate Hex-Rays for
providing high-quality signatures for IDA Pro and thank them for
sharing their research and tools with the community.

To learn more about the pattern and signature file generation check
out the siglib
repository. The FLAIR utilities are available in the protected
download area on Hex-Rays’ website.

Rule Updates

Since the initial release, the community has more than doubled the
total capa rule count from 260 to over 570 capability
detection rules
! This means that capa recognizes many more
techniques seen in real-world malware, certainly saving analysts time
as they reverse engineer programs. And to reiterate, we’ve surfed a
wave of support as almost 30 colleagues from a dozen organizations
have volunteered their experience to develop these rules. Thank you!

Figure 8 provides a high-level overview of capabilities capa
currently captures, including:

  • Host Interaction describes program functionality to interact
    with the file system, processes, and the registry
  • Anti-Analysis describes packers, Anti-VM, Anti-Debugging, and
    other related techniques
  • Collection describes functionality used to steal data such as
    credentials or credit card information
  • Data Manipulation describes capabilities to encrypt, decrypt,
    and hash data
  • Communication describes data transfer techniques such as
    HTTP, DNS, and TCP

Figure 8: Overview of capa rule categories

More than half of capa’s rules are associated with a MITRE ATT&CK technique
including all techniques introduced in ATT&CK version
that lie within capa’s scope. Moreover, almost half of the capa
rules are currently associated with a Malware Behavior
(MBC) identifier.

For more than 70% of capa rules we have collected associated
real-world binaries. Each binary implements interesting capabilities
and exhibits noteworthy features. You can view the entire sample
collection at our capa test files
GitHub page
. We rely heavily on these samples for developing and
testing code enhancements and rule updates.

Python 3 Support

Finally, we’ve spent nearly three months migrating capa from Python
2.7 to Python 3. This involved working closely with vivisect and we would
like to thank the team for their support. After extensive testing and
a couple of releases supporting two Python versions, we’re excited
that capa 2.0 and future versions will be Python 3 only.


Now that you’ve seen all the recent improvements to capa, we hope
you’ll upgrade to the newest capa version right away! Thanks to
library function identification capa will report faster and more
relevant results. Hundreds of new rules capture the most interesting
malware functionality while the improved capa explorer plugin helps
you to focus your analysis and codify your malware knowledge while
it’s fresh.

Standalone binaries for Windows, Mac, and Linux are available on the
capa Releases
. To install capa from PyPi use the command
pip install flare-capa. The source code is
available at our capa GitHub
. The project page on GitHub contains detailed documentation,
including thorough installation
instructions and a walkthrough of capa
. Please use GitHub to ask
questions, discuss ideas, and submit issues.

We highly encourage you to contribute to capa’s rule corpus. The
improved IDA Pro plugin makes it easier than ever before. If you have
any issues or ideas related to rules, please let us know on the GitHub repository.
Remember, when you share a rule with the community, you scale your
impact across hundreds of reverse engineers in dozens of organizations.