Note: The job is a remote job and is open to candidates in USA. Horizon3.ai is a fast-growing, remote cybersecurity company focused on proactive security solutions. They are seeking a Senior Software Engineer specializing in browser automation to enhance their autonomous web application penetration testing capabilities, ensuring effective navigation and interaction with modern web applications.
Responsibilities
- Help us grow and harden our browser automation and crawling engine, which is the layer that discovers, navigates, and interacts with target applications before and during an autonomous pentest
- Advance our browser-driven crawler using Playwright and Stagehand
- Tackle the gnarly realities of modern web apps: SPA routing and hydration timing, authenticated sessions, multi-step flows, file uploads, WebSocket/Socket.IO traffic, infinite scroll, and crawler traps
- Extend our agentic login and authentication capabilities, including complex auth flows, MFA/TOTP, and credentialed access reliable enough to run unattended against customer environments
- Improve crawl coverage, determinism, and throughput. This involves endpoint and parameter discovery, dedupe, queueing, and state management, while keeping everything production-safe and side-effect-aware
- Help draw the line between deterministic automation and LLM-driven navigation, applying models surgically rather than as a default, and keeping the system fast, debuggable, and cheap to run
- Collaborate with the attack-team engineers who consume your crawl output, and help shape the graph-backed application map the rest of the pipeline depends on
Skills
- Experience building production software, with deep, hands-on experience in browser automation (Playwright, Puppeteer, or Selenium) against real, non-trivial web applications
- Strong TypeScript / Node.js skills and comfort living inside the headless-browser stack, including Chromium internals, the Chrome DevTools Protocol, network interception, the DOM, and JS execution contexts
- A track record of taming flaky, stateful, JavaScript-heavy apps. You've fought SPA timing, authentication, and anti-automation defenses and won
- Solid instincts for distributed/concurrent systems: queues, backpressure, retries, idempotency, and running many browser sessions reliably at scale
- A bias toward determinism and debuggability, and the judgment to reach for an LLM only when a deterministic approach genuinely can't do the job
- Ownership mentality: you are comfortable taking a critical subsystem from 'works' to 'works unattended, at scale, against someone else's production environment.'
- Experience with agentic browser frameworks (Stagehand, Browser Use, or similar) or building LLM-in-the-loop automation
- Background in web application security or offensive tooling — familiarity with broken access control, IDOR/BOLA, SQLi, XSS, SSRF, or SSTI in the wild
- Familiarity with graph data models (e.g., Neo4j) for representing application structure
- Experience with large-scale crawling, endpoint discovery (e.g., parsing/analyzing client-side JS), or session/credential management for automated access
- Comfort working in an environment where correctness against a live customer system is a hard, non-negotiable constraint
Benefits
- Equity package in the form of stock options
- Health, vision & dental insurance for you and your family
- A flexible vacation policy
- Generous parental leave
- Hybrid & Remote Work: We embrace a mix of remote and hybrid work models depending on role and location, including our Chicago office, where some roles require regular in-office presence
Company Overview