fix: resolve privilege escalation vulnerability via mass assignment (#1851)

[atul-upadhyay-7]

Summary

This PR fixes the Privilege Escalation via Mass Assignment vulnerability described in #1851. The fix ensures that standard users cannot escalate their roles by injecting privileged fields (e.g., "is_admin": true` via the Supabase client to grant themselves administrative privileges, bypassing all authorization checks.

Changes

Database Layer (RLS Hardening)

• New migration (20260727000000_fix_mass_assignment_privilege_escalation.sql):
  • Added to the RLS WITH CHECK clause, preventing users from self-promoting to admin
  • Added defense-in-depth trigger (prevent_privilege_escalation) that unconditionally blocks direct modification of and columns
  • Only SECURITY DEFINER functions (gamification RPCs) can modify these columns

Backend Layer (Server-Side Validation)

• New validation schema (backend/validation/schemas.js):
  • Added with Zod validation that rejects unknown keys
• New secure endpoint (backend/routes/users.js):
  • Added with strict field whitelisting
  • Only allows safe fields: name, bio, skills, avatar_url, interests, teach_subjects, learn_subjects
  • Explicitly blocks and logs privilege escalation attempts (is_admin, is_mentor, points, role, etc.)
  • Enforces ownership check (users can only update their own profile)

Frontend Layer (Defense-in-Depth)

• Updated src/pages/EditProfile.tsx: Uses new secure backend endpoint instead of direct Supabase client
• Updated src/pages/Profile.tsx: Uses new secure backend endpoint instead of direct Supabase client

Security Architecture

The fix implements defense-in-depth across three layers:

1. Database RLS — WITH CHECK clause prevents privileged column mutation
2. Database Trigger — Unconditionally blocks is_admin/is_mentor changes from direct SQL
3. Backend Validation — Zod + field whitelisting rejects unknown/restricted fields
4. Backend Logging — All privilege escalation attempts are logged for audit

Testing

• TypeScript compilation: ✅ Clean (no errors)
• Security tests: ✅ 5/5 passed
• All unit tests: ✅ 37/37 passed

Fixes #1851

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📥 Commits

Reviewing files that changed from the base of the PR and between 3565841 and b15d90a.

📒 Files selected for processing (5)

• backend/routes/users.js
• backend/validation/schemas.js
• src/pages/EditProfile.tsx
• src/pages/Profile.tsx
• supabase/migrations/20260727000000_fix_mass_assignment_privilege_escalation.sql

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• Create PR with unit tests

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