Missing WHERE Clause in B2B SaaS Can Leak One Customer's Data to Another

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A single missing WHERE clause in a database query can expose one tenant's private data to another in multi-tenant B2B SaaS applications, even when users are properly authenticated. A developer building a platform called BootSaaS outlined how authentication alone does not guarantee tenant isolation, since a valid logged-in user can still receive another organization's records. To address this, the developer chose a schema-per-tenant architecture using Spring Boot, PostgreSQL, and Liquibase, keeping each customer's business data in separate tables. This approach avoids relying on every query remembering a tenant filter, which is the core vulnerability in shared-table designs. The article also contrasts three isolation strategies — shared tables with row-level filtering, schema-per-tenant, and database-per-tenant — noting that database-per-tenant introduces significant operational overhead, especially around connection pool management at scale.

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