Senior Software Engineer, Browser Automation

The role

You will grow and harden a browser automation and crawling engine for an autonomous web application penetration tester. This involves advancing a browser-driven crawler and extending agentic login and authentication capabilities.

What you will do

• Help grow and harden the browser automation and crawling engine
• Advance the browser-driven crawler using Playwright and Stagehand
• Tackle modern web app realities like SPA routing, authenticated sessions, and WebSocket traffic
• Extend agentic login and authentication capabilities including MFA/TOTP
• Improve crawl coverage, determinism, and throughput
• Apply LLMs surgically to maintain a fast and debuggable system
• Collaborate with attack-team engineers to shape the graph-backed application map

Required qualifications

• Experience building production software
• Deep, hands-on experience in browser automation (Playwright, Puppeteer, or Selenium) against real, non-trivial web applications
• Strong TypeScript / Node.js skills
• Comfort with the headless-browser stack including Chromium internals, Chrome DevTools Protocol, network interception, the DOM, and JS execution contexts
• Track record of taming flaky, stateful, JavaScript-heavy apps
• Solid instincts for distributed/concurrent systems including queues, backpressure, retries, and idempotency
• Bias toward determinism and debuggability
• Ownership mentality

Nice to have

• Experience with agentic browser frameworks (Stagehand, Browser Use, or similar)
• Building LLM-in-the-loop automation
• Background in web application security or offensive tooling
• Familiarity with graph data models (e.g., Neo4j)
• Experience with large-scale crawling, endpoint discovery, or session/credential management
• Hacking on Playwright or Puppeteer internals or contributing to the core
• Building browser automation at extreme scale
• Outmaneuvering WAFs, anti-bot defenses, and fingerprinting mechanisms
• Offensive security mindset
• Battle-tested experience with LLMs in production

Benefits

• Equity package in the form of stock options
• Health, vision & dental insurance for you and your family
• Flexible vacation policy
• Generous parental leave

About the company

Horizon3 is a fast-growing, remote cybersecurity company dedicated to enabling organizations to proactively find, fix, and verify exploitable attack vectors. Their flagship product is the NodeZero platform.