The Open-Source Captcha for Privacy-First Bot Protection
ALTCHA is a free, open-source captcha that stops bots and spam without cookies, tracking, or annoying puzzles. Self-host the MIT-licensed core today, or scale up to Cloud, Sentinel, or the WordPress plugin whenever you need more.
A Captcha You Can Own, Audit, and Self-Host
Most captchas are closed-source services that track visitors across the web and route their data through third-party servers. ALTCHA takes a different approach: the widget, verification server, and integration libraries are all open source and released under the permissive MIT license, so you can read the code, audit it, and run the entire stack on your own infrastructure.
Instead of distorted images or traffic-light puzzles, ALTCHA relies on a self-hosted proof-of-work challenge — a small cryptographic computation the visitor's browser solves automatically in the background. There's nothing for humans to click or decipher, no cookies are set, and no personal data ever leaves your server.
With no visual puzzle to solve, ALTCHA is accessible by default — WCAG 2.2 AA conformant, fully keyboard-navigable, and screen-reader labelled, with a built-in audio alternative for the rare case an interactive challenge is needed.
Because it’s open source, ALTCHA works with any stack: drop the widget into a React, Vue, Svelte, or plain HTML page, and verify the result with official server libraries for Node.js, PHP, Python, Go, Java, Rust, Ruby, and more.
Open, accessible, and free
- WCAG 2.2 AA accessible, no visual puzzles to solve
- No cookies, tracking, or data leaving your server
- No usage limits, API keys, or sign-up required
- Runs entirely on your own infrastructure
- MIT licensed
Built for Teams Who Value Control
Self-hosting the open-source core gives you full ownership of your data and your uptime — with no recurring fees.
Privacy by design
- No cookies or fingerprinting
- No visitor data leaves your server
- GDPR, CCPA & DPDPA friendly
Accessible to everyone
- WCAG 2.2 AA conformant
- Full keyboard navigation
- Screen-reader labelled
- No visual puzzles
Full ownership
- MIT licensed, source available
- Self-hosted verification server
- No vendor lock-in
Developer friendly
- Widgets for every major framework
- Server libraries in 10+ languages
- Simple, well-documented API
From Open Source to Fully Managed
Start free and self-hosted. Upgrade to Cloud, Sentinel, or the WordPress plugin whenever you need adaptive protection, dashboards, or managed infrastructure.
Open Source
The free, MIT-licensed core. Widgets and server libraries you host and run yourself.
- MIT licensed
- Proof-of-work verification
- No sign-up or API keys
- Community support
ALTCHA Cloud
Zero infrastructure — hosted in the EU and powered by Sentinel-grade protection.
- Adaptive, risk-based verification
- Real-time threat intelligence
- EU-only endpoints
- Email support
ALTCHA Sentinel
The full protection engine behind Cloud — deployed on your own infrastructure.
- Dashboard & analytics
- Adaptive risk scoring
- Rate limiting & security rules
- Runs on Docker, Kubernetes, AWS & Azure
WordPress Plugin
Self-contained protection for WordPress logins, comments, and forms.
- Protects 22+ popular plugins
- No external services
- Professional plan from € 18.90/mo
Open Source vs. Cloud vs. Sentinel
All three run on the same privacy-first, WCAG-accessible proof-of-work engine. Cloud and Sentinel add adaptive risk scoring, dashboards, and threat intelligence on top.
| Open Source | Cloud | Sentinel | |
|---|---|---|---|
| No cookies or tracking | ✓ | ✓ | ✓ |
| GDPR & CCPA compliant | ✓ | ✓ | ✓ |
| WCAG 2.2 AA accessible | ✓ | ✓ | ✓ |
| Proof-of-work verification | ✓ | ✓ | ✓ |
| Self-hosted, full data ownership | ✓ | ✗ | ✓ |
| Adaptive, risk-based challenges | ✗ | ✓ | ✓ |
| Dashboard & analytics | ✗ | ✓ | ✓ |
| Real-time threat intelligence | ✗ | ✓ | ✓ |
| Rate limiting & security rules | ✗ | ✓ | ✓ |
| Priority support | ✗ | ✓ | ✓ |
The WordPress plugin includes open-source-equivalent protection out of the box, with Sentinel-level features available on the Professional plan. See the full pricing comparison for exact limits.
Common Questions About Open-Source ALTCHA
What is an open-source captcha?
An open-source captcha is a bot-protection widget whose source code is publicly available, so anyone can inspect, modify, and self-host it. ALTCHA's widget, verification server, and client libraries are released under the MIT license on GitHub, meaning there's no vendor to trust with your visitors' data — you run the whole stack yourself.
Is ALTCHA really free to use?
Yes. The open-source core is free forever, with no usage limits, API keys, or sign-up required. You only pay if you choose a managed option like ALTCHA Cloud, self-hosted Sentinel, or the WordPress Professional plan for extra features such as dashboards and adaptive risk scoring.
What license is ALTCHA released under?
The open-source widget and server libraries are MIT licensed, so you're free to use, modify, and redistribute them — including in commercial products — without royalties or attribution requirements.
How is open-source ALTCHA different from Cloud and Sentinel?
The open-source core handles proof-of-work verification: it blocks bots without any dashboard, analytics, or adaptive risk scoring. ALTCHA Cloud and Sentinel run the same verification engine but add a hosted dashboard, real-time threat intelligence, rate limiting, and adaptive challenges that escalate only when risk signals demand it. Cloud is fully managed by us in the EU; Sentinel is the same engine deployed on your own infrastructure.
Does the WordPress plugin use the open-source core?
Yes. The free WordPress plugin builds on the same proof-of-work verification as the open-source core, pre-integrated with WordPress login, registration, comments, and 22+ popular form plugins. A paid Professional plan adds extra sites and priority support.
Is self-hosted ALTCHA GDPR compliant?
Yes. Because the open-source core sets no cookies and sends no visitor data to third parties, it doesn't require a cookie-consent banner and is compliant with GDPR, CCPA, and similar privacy regulations by design.
Is the open-source captcha accessible (WCAG)?
Yes. ALTCHA is WCAG 2.2 AA conformant out of the box — fully keyboard-navigable and screen-reader labelled, with no distorted images or puzzles to solve, and a built-in audio alternative for the rare case an interactive challenge is triggered. This applies across the open-source core, Cloud, Sentinel, and the WordPress plugin.
Ready to Stop Bots Without Compromising Privacy?
Install the open-source widget in minutes, or explore Cloud, Sentinel, and WordPress if you need more.