Senior Software Engineer, Browser Automation

Role Description

We're building an autonomous, black-box web application penetration tester. It crawls and attacks real production websites the way a skilled human pentester would, finding broken access control, injection, XSS, and more, under a strict production-safe, no-false-positives mandate. The hardest part of that job isn't the exploitation. The hardest part is reliably driving a real browser through messy, modern web apps at scale: logging in, navigating SPAs, surviving anti-bot defenses, and mapping every reachable surface without getting stuck or causing harm. That's the engine you'd own.

Essential Functions

• 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.

Qualifications

• 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."

Desired/Nice to Have

• 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.

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 know exactly how systems break under pressure and have the war stories to prove it.
• 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. You understand the engineering trade-offs: knowing when AI is an asset and when it introduces unacceptable latency or nondeterminism compared to a deterministic script.

Benefits

• 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.

Compensation and Values

At Horizon3, we believe that our people are our greatest asset, and our compensation philosophy reflects this core value. We are committed to fostering an environment where all employees feel valued, respected, and rewarded for their contributions. Our compensation structure is designed to be fair, competitive, and transparent, ensuring that every team member is recognized and compensated equitably across roles, levels, and locations.

In accordance with various State’s transparency regulations, we provide the following salary range information for this position:

• Base salary range: $169,000 - $208,000 annually. The exact salary will be determined based on the selected candidate’s location, qualifications, experience, and relevant skills.
• Additional compensation: All full-time roles are eligible for an equity package in the form of stock options.

You Belong Here

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 a workplace where everyone feels 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.

Our commitment to diversity and inclusion means we strive to attract, develop, and retain a workforce that reflects the varied communities we serve. We believe that diverse perspectives drive innovation and strengthen our ability to create cutting-edge cybersecurity solutions. At Horizon3, every team member is valued and supported in an environment that encourages personal and professional growth.

We welcome candidates from all backgrounds and experiences, and we encourage all qualified individuals to apply. Come be a part of Horizon3, where your unique contributions are recognized, and your potential is limitless.

Other Duties

Please note this job description is not designed to cover or contain a comprehensive listing of activities, duties or responsibilities that are required of the employee. Duties, responsibilities, and activities may change at any time with or without notice.

Application Note

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