epocsoft

Overview

Pegasus is a specialized Windows malware emulation framework designed to execute binary samples within a modeled Windows runtime environment, eliminating the need for a full virtual machine. By emulating core operating system components—such as APIs, process and thread behavior, the filesystem, the registry, and network activities—Pegasus enables malicious binaries to traverse realistic execution paths without the overhead or detection risks of traditional hypervisor-based sandboxes.
The primary goal of Pegasus is to provide high-resolution emulation of the Windows operating system for dynamic malware analysis on x86 and amd64 platforms. It is architected specifically around the needs of malware reverse engineering, with emulation and API support continuously driven by current malware trends. The framework offers the community a lightweight, scalable means to extract indicators of compromise (IOCs) without requiring additional tooling or complex configurations.


Key Features

  • Malware-First Architecture

Pegasus is built from the ground up to emulate Windows behaviors most relevant to modern malware families, rather than aiming for complete OS emulation.

  • Trend-Driven API Support

API handlers are prioritized based on real-world malware usage, ensuring rapid coverage for emerging threats.

  • Zero-Code Configuration

The emulation environment is fully configurable through settings files, requiring no modifications to the framework’s source code.

  • Automated Analysis Pipeline

Samples can be processed in batch mode, with structured reports generated for post-processing, enabling high-volume triage.

  • Pluggable Emulation Engine

While Pegasus currently uses the Unicorn engine (a QEMU-based CPU emulator) for x86 and amd64 instruction emulation, an abstraction layer allows future support for alternative emulation backends.


Background: Dynamic Analysis Challenges

Dynamic analysis of Windows malware is essential for understanding how a sample interacts with the Windows API and for extracting host- and network-based IOCs. Traditional approaches fall into two categories:

  • Automated sandboxing – Samples are executed in a monitored VM. This approach is effective but resource-intensive, difficult to scale on demand, and increasingly prone to sandbox evasion techniques.
  • Manual debugging – Analysts step through code to reveal hidden paths, but this is time-consuming and not suitable for large-scale triage.

Emulation vs. Hypervisor Sandboxing

Advantages of emulation:

  • Noise reduction – Only activity explicitly coded by the malware author or compiled into the binary is recorded. API calls from system libraries or hypervisor hooks are not interleaved, reducing attribution ambiguity.
  • Total instrumentation – CPU instruction streams are fully observable, and control flow can be manipulated to maximize code coverage.
  • Lightweight scaling – Emulation sessions start and stop quickly, enabling high-throughput analysis without VM provisioning overhead.

Disadvantages of emulation:

  • OS responsibility shift – The emulator must faithfully provide expected inputs, outputs, and side effects for every API call, memory access, and exception the malware expects from a genuine Windows system.
  • Ongoing maintenance – As Windows evolves and new malware techniques emerge, the emulation surface must be continuously updated.

Pegasus acknowledges that full OS sandboxing will always be required for certain samples—generic emulation of all Windows behaviors is unfeasible. However, by focusing on specific malware families and common execution patterns, Pegasus significantly reduces the need to reverse engineer every variant manually. Automated triage reports generated by Pegasus are often sufficient for initial assessment, allowing analysts to reserve deep dives for only the most complex or novel threats.


Technical Architecture

CPU Emulation

Pegasus currently relies on Unicorn, a lightweight, multi-architecture CPU emulator based on QEMU. Unicorn provides instruction-level emulation for x86 and amd64, with full support for hooks, memory introspection, and exception handling.

API Handling

Windows API calls are intercepted and processed by C++ API handlers. Each handler is responsible for emulating the expected behavior of a specific API, including:

  • Return values (success/failure codes)
  • Output parameters (e.g., buffers filled with data)
  • Side effects (e.g., modifying emulated registry keys, creating files, spawning threads)

To define a new API handler, only three pieces of information are required:

  • API name (e.g., CreateFileW)
  • Argument count (number of parameters)
  • Calling convention (defaults to stdcall if not specified; cdecl, fastcall, etc., are also supported)

If an API call is not yet implemented, Pegasus logs the unsupported API and attempts to continue execution at the next entry point, preventing the sample from crashing fatally.
Emulated Environment Components

  • Process and thread management – Context switching, thread local storage (TLS), and exception dispatch
  • Filesystem – Virtual file hierarchy with configurable paths, permissions, and content
  • Registry – Key-value store mimicking Windows registry hives (HKLM, HKCU, etc.)
  • Network – Simulated socket operations, DNS resolution, and configurable network responses

Ongoing Development

The Pegasus project is continuously evolving. Future efforts focus on:

  • Expanding API coverage for newly observed malware families
  • Improving fidelity of emulated Windows behaviors (e.g., more realistic timing, error codes, and side effects)
  • Enhancing reporting formats for integration with downstream analysis pipelines
  • Supporting additional emulation engines through the abstraction layer