Cossack Labs

This position is open exclusively to Ukrainian residents currently living in Ukraine (preferably in Kyiv or Lviv).

Our product team is looking for a full-stack Python developer with hands-on experience in building UI applications, ideally someone open to learning or already exploring Go or Rust. You’ll help create tools used by engineers worldwide and contribute to mission-critical projects. We’re primarily looking for mid-level specialists. However, if you’re more senior and have a background in cryptography, security, or systems programming, we’d be happy to talk.

Requirements:

• Python 3, Django or aiohttp 3+ years
• Asyncio, queues, synchronisation
• Django ORM or Sqlalchemy Core
• PostgreSQL, raw SQL
• JavaScript, React 2+ years
• HTML5 / CSS3
• Docker, docker compose
• Linux, bash
• Understanding of GOF, SOLID, Dependency Injection, Clean Architecture principles, Modular Monoliths
• Understanding of asynchronous and multi-threaded programming: asyncio, threading, multiprocessing
• Good spoken and written English (B2+) for team communication and code documentation

Nice to have:

• Golang
• Typescript
• OAuth2
• Prometheus
• UIkit
• RabbitMQ
• gRPC
• Datatables (javascript)
• GitHub Actions

Main responsibilities:

• Take ownership of developing user-facing features with a focus on responsive and intuitive design
• Build efficient and reusable front-end systems to power our web applications
• Write clean, maintainable, and well-documented code
• Participate in code reviews and contribute to both individual and team growth

What to expect from us:

• We build applications for clients, write code for developers, and document how it all works — for both
• Everyone on the team is happy to help, but we also respect each other’s time and know how to find answers ourselves
• Tech writing isn’t our favourite task, but we do create solid technical documentation when needed, and we’re lucky to have a tech writer to help with that
• We prefer doing things right the first time over patching things endlessly later
• We value healthy perfectionism, a sense of beauty in engineering, and a good sense of humour

Hiring process:

• Resume review — up to 5 business days
• Test task — estimated time 3-4 hours
• Introductory meeting with the Head of development & solution engineering
• Technical interview with several team members
• Offer discussion

What’s in it for you?

• Competitive compensation with a flexible and clear bonus scheme
• Paid vacation — 21 business days per calendar year
• Paid sick leaves
• Hybrid work model: this position allows for a combination of in-office and remote work as needed
• Combining technologies: hardware engineering, software engineering, cryptography, information security
• You will work with people deeply interested in security engineering, you will learn a lot
• Reasonable time budgets and an attitude to build things well — we prioritise building for decades, rather than just until the next release
• Conferences, books, courses — we encourage learning and sharing with the community. Our team members share a a lot in talks, workshops, and blog posts
• Public track record in the open-source aspect of our products.

Our software is well-known amongst security-aware teams, recommended by OWASP, and popular for easily solving complicated security challenges. Apart from building “off-the-shelf” solutions, we design custom security solution for novel problems. We work in the B2B space, with customers such as power grid operators, payment processors, legal companies, and million-user customer applications. We cater to young ambitious startups and well-established enterprises, that use our software and solutions as a core part of their security arsenal.

This position is open exclusively to Ukrainian residents within Ukraine (preferably Kyiv).
 

We are hiring a Senior Node.js Engineer who will primarily build and evolve our web platform while serving as the first-line responder during business hours, seven days per week. Most days pass without incidents and weekend requests are rare; your time will largely focus on development across Node.js, React, and PostgreSQL.
 

Support profile:

• Coverage: business hours, seven days per week; weekend requests are rare.
• Expectation: prompt acknowledgment, swift triage, and practical restoration; escalate when needed.
• Reality: frontline duties are light; the majority of your time is dedicated to development.
   

Main responsibilities:

• Contribute the majority of your time to product development: design, implementation, and testing.
• Deliver high-quality Node.js services and React UI changes; write maintainable, well-tested code.
• Design SQL beyond ORM abstractions; analyze queries, optimize performance, and evolve schemas.
• Improve reliability by creating runbooks, post-incident reviews, and automation for recurring tasks.
• Own rapid first response during business hours via instant messenger and an emergency phone line.
• Triage, reproduce, and diagnose issues across the stack; restore service quickly where possible.
• Apply safe workarounds, configuration tweaks, or hotfixes; execute rollbacks when needed.
• Escalate efficiently to the development team with clear context, logs, impact, and proposed next steps.
• Communicate status and timelines to internal stakeholders and customer contacts.
• Enhance monitoring and alerting; instrument services for actionable logs, metrics, and traces.
   

Requirements:

• Senior-level experience building production Node.js applications.
• React familiarity for practical UI issues diagnosis.
• PostgreSQL expertise and strong SQL skills beyond ORM usage, including query design and tuning.
• Hands-on experience with incident triage, root-cause analysis, and production debugging.
• Comfortable with Linux servers and on-prem fundamentals (DNS, TLS, certificates, basic networking).
• Proficient with Git workflows and CI/CD practices.
• Familiarity with observability (logs, metrics, traces) and structured troubleshooting.
   

Nice to have:

• Python or Go experience for tooling, services, or automation.
• Solid understanding of Docker for local development and packaging.
• Knowledge of reverse proxies/load balancers (Nginx, HAProxy, Traefik).
• Exposure to container orchestration (Docker Swarm or Kubernetes) and IaC (Terraform/Ansible).
• Practical experience in diagnosing infrastructure problems: resources congestions, network connectivity issues, PKI and other protocol-level security problems.
• Experience with Sentry, Prometheus/Grafana, ELK/OpenSearch, or OpenTelemetry.
   

Hiring Process:

• Resume review — up to 5 business days.
• Introductory meeting with the Systems/Software Architect.
• Technical interview with several team members.
• Offer discussion.
   

We Offer:

• Competitive compensation.
• Hybrid work model: combination of in-office and remote work as needed.
• Paid vacation — 21 business days per year.
• Paid sick leave.
• Exposure to intersecting domains: software development, information security, and cloud/on-prem infrastructure engineering.
• Experience in mission-critical projects.