pytm

Overview

pytm is an open-source Python framework for defining threat models as code and generating diagrams, findings, reports, and slides. It suits developers and security practitioners who want threat modeling in a Python-based workflow, with room to customize threats and report output. Its code-first approach is flexible, but diagram generation depends on external tools and the project warns that a changeable threats file can create a code-execution risk.

Key features

Models are built from Python elements such as actors, servers, processes, data stores, data flows, and trust boundaries. From that architecture, pytm can produce data-flow diagrams, sequence diagrams, and relevant threat findings. This makes it a natural fit for teams that want the model alongside development work; readers who prefer a graphical modeling interface may find the code-based method less suitable.

Diagram generation uses Graphviz DOT for data-flow diagrams and PlantUML for sequence diagrams. Templates can combine diagrams and findings in reports, while a RevealMD template produces slides that can be presented in a browser. Models can also be loaded from JSON through pytm's Python function, and custom threat files can change finding attributes such as severity, CVSS score, and response text. That flexibility helps teams adapt findings to their own process, but puts more responsibility on them to maintain the model and templates.

The project documents container and Devbox workflows for dependencies. Its example report command uses Pandoc, adding another tool to account for when generating reports. The README cautions that strings in threats.json are evaluated with eval(); protect the file's permissions so an attacker cannot alter those strings and execute code.

Pricing

Open-source pytm: free. The free plan is the open-source framework, with no paid tier described. It suits developers who can run and maintain the tooling themselves. Python 3.11 or later is required, and diagram generation requires Graphviz and Java/PlantUML.

Platforms

pytm is listed for Linux, macOS, and Windows. Its documented Windows setup uses Devbox through WSL. The stated platform requirements specify Linux or macOS, so Windows users should expect to follow that WSL workflow rather than a native setup. The requirements include Python 3.11 or later, Graphviz, Java OpenJDK 10 or 11, and plantuml.jar.

Who it's for

pytm is best for developers and security practitioners who want threat models represented in Python and generated artifacts integrated into a developer-centric process. Custom threats and templates suit teams prepared to shape their own findings and reports. It is a weaker fit for people seeking a graphical-first modeling experience or teams that do not want to manage Python and diagram-generation dependencies.

Pros and cons

• Pros: Python-defined architecture can generate diagrams and relevant threat findings from one model, which suits code-oriented workflows.
• Pros: Custom threats, adjustable finding attributes, report templates, and browser-presentable slides give teams control over how they prioritize and communicate results.
• Cons: Diagram generation requires Graphviz and Java/PlantUML, and reports may also involve Pandoc; setup takes more than installing a standalone application.
• Cons: Windows usage is documented through WSL, while the stated requirements name Linux or macOS.
• Cons: The eval() behavior for strings in threats.json makes file permissions a security concern.

Alternatives

CAIRIS is another free option, available across Linux, macOS, Windows, web, API, and self-hosted platforms.

OWASP Threat Dragon is a free open-source alternative for Linux, macOS, Windows, web, and self-hosted use.

ThreatForge is free and open source under Apache 2.0, with no account required and a web option alongside desktop platforms.

IriusRisk offers a free Community Edition for up to three active threat models and one user with limited collaboration, including templates and libraries and XML diagram export.

Threagile is a free, MIT-licensed open-source toolkit, with Linux, API, web, and self-hosted platform options.

ThreatModeler Nexus has a free Community Edition aimed at practitioners, students, developers, architects, and security teams.

ThreatOpus may suit teams looking for a paid collaborative service: its Starter plan costs £129.99 a month and includes 15 users, 10 team workspaces, 50 threat modeling generations per month, 10 repositories, and SCM and CI provider integrations.

ThreatTree offers a free plan capped at three forests, three DFDs per forest, and five attack trees per DFD; its Pro plan is $29 a month, billed per user monthly.

For more options, see Threat Modeling Software.

Verdict

Choose pytm if you want threat modeling in Python, with generated diagrams and findings plus customizable reports and slides. Its strongest appeal is the ability to treat an architecture as code and tailor the resulting output. Look elsewhere if you want a graphical modeling workflow or need to avoid external diagram tools and careful protection of the threats file.