Fractional Staff Engineer / Software Architect — Secure Multi-Tenant SaaS

Prox Technologies is building ProxBooks, a multi-tenant B2B SaaS platform for U.S. accounting firms. The role will independently assess the platform’s architecture and implementation for application and product security, including threat modeling, tenant isolation, cloud exposure, API security, AI-agent risks, and remediation validation.

Responsibilities:

• Perform threat modeling
• Review authentication and authorization architecture
• Review tenant isolation
• Review PostgreSQL security and RLS strategy
• Review file-upload architecture
• Review secrets management
• Review cloud/network exposure
• Review API security
• Review AI-agent permissions and attack surfaces
• Review logging and sensitive-data exposure
• Review backup and recovery controls
• Review CI/CD security
• Review critical pull requests
• Define security acceptance tests
• Validate remediation work

Requirements:

• We are looking for a senior Application Security / Product Security Engineer to independently review our architecture and implementation
• We expect practical knowledge of:
• OWASP ASVS
• OWASP API Security
• IDOR / BOLA
• Least privilege
• RBAC
• Tenant isolation
• Secure file uploads
• Signed URLs
• Malware scanning
• Secrets management
• Encryption
• Audit logging
• Webhook security
• Rate limiting
• Supply-chain security
• Cloud IAM
• Secure CI/CD
• Please do not send a generic cybersecurity proposal
• We are specifically looking for hands-on application/product security experience
• Experience with financial services, accounting, tax software, fintech, healthcare, or similarly sensitive systems is highly desirable
• Experience with AI/LLM security and prompt-injection defenses is a plus