Senior Software Engineer, Browser Automation

About the role

We are seeking a talented engineer to join our team as a browser automation specialist. Your primary responsibility will be to build and improve our browser automation engine, which powers our autonomous pentesting platform.

Responsibilities

• Advance our browser-driven crawler using Playwright and Stagehand.
• Tackle the complexities of modern web apps, including SPA routing, authenticated sessions, and multi-step flows.
• Extend our agentic login and authentication capabilities, ensuring reliability and security for production environments.
• Improve crawl coverage, determinism, and throughput by discovering endpoints, managing parameters, and maintaining production safety.
• Collaborate with the attack team to refine the application map and ensure seamless integration with the rest of the pipeline.

Requirements

• Deep, hands-on experience in browser automation (Playwright, Puppeteer, or Selenium) against real, non-trivial web applications.
• Strong TypeScript / Node.js skills and comfort with the 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, including SPA timing, authentication, and anti-automation defenses.
• Experience with distributed/concurrent systems, including queues, backpressure, retries, idempotency, and reliable operation at scale.
• Ownership mentality: a willingness to take a critical subsystem from "works" to "works unattended, at scale, against someone else's production environment."

Competencies/Qualifications

• Experience with agentic browser frameworks (Stagehand, Browser Use, or similar) or building LLM-in-the-loop automation.
• Familiarity with graph data models (e.g., Neo4j) for representing application structure.
• Experience with large-scale crawling, endpoint discovery, session/credential management, or automated access.
• Battle-tested experience with LLMs in production and a solid understanding of their engineering trade-offs.

Desired/Nice to Have

• Experience with Playwright, Puppeteer, or Selenium.
• Background in web application security or offensive tooling.
• Familiarity with broken access control, IDOR/BOLA, SQLi, XSS, SSRF, or SSTI in the wild.
• Comfort working in an environment where correctness against a live customer system is a hard, non-negotiable constraint.

What Makes You Stand Out

• You’ve gone beyond using tools like Playwright or Puppeteer to actually hacking on their internals or contributing to the core.
• You’ve built browser automation at extreme scale, handling thousands of sessions against hostile, heavily-defended targets.
• You’ve successfully outmaneuvered sophisticated WAFs, anti-bot defenses, and fingerprinting mechanisms in production environments.
• You have an offensive security mindset: you don’t just navigate a web app; you actively map its attack surface and hunt for unreachable paths.
• You have battle-tested experience with LLMs in production and a deep understanding of their engineering trade-offs.

Perks of Horizon3.ai

• Inclusive Team: We value diversity and promote an inclusive culture where everyone can thrive.
• Growth Opportunities: Be part of a dynamic and growing team with numerous career development opportunities.
• Innovative Culture: Work in a collaborative environment that encourages creativity and out-of-the-box thinking.
• 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.
• Competitive Compensation: We offer competitive salary, equity, and benefits. Our benefits include health, vision & dental insurance for you and your family, a flexible vacation policy, and generous parental leave.

Career Development

• Be part of a dynamic and growing team with numerous career development opportunities.

Compensation And Values

• Base salary range: $169,000 - $208,000 annually.
• Additional compensation: All full-time roles are eligible for an equity package in the form of stock options.

Other Information

• Our benefits include health, vision & dental insurance for you and your family, a flexible vacation policy, and generous parental leave.
• Horizon3 is not just an equal opportunity employer - we are a community that values diversity, equity, and inclusion as fundamental principles of our culture and success.
• We are dedicated to fostering an environment where all employees feel welcome and respected, regardless of race, color, religion, sex, national origin, age, disability, veteran status, sexual orientation, gender identity or expression, genetic information, marital status, hair length or any other legally protected status by law.