Appsec

Fourth in a series on building out-of-band application security testing (OAST) infrastructure from scratch. Earlier posts went down to the wire (JNDI, DNS, the sandbox). This one is about a quieter risk that kills more SaaS products than any exploit: one missing WHERE clause. In a multi-tenant app, the entire security model often reduces to a single sentence: every database read that returns user-owned data is filtered by the current user’s id. Get it right everywhere and tenants are isolated. Forget it in one endpoint — one db.query(Modifier).filter(Modifier.id == requested_id) without the owner_user_id check — and any user can read any other user’s data by guessing an integer. It’s the most common serious bug in SaaS, and it’s a bug of omission, which is exactly the kind code review is worst at catching. ...

Third in a series on building out-of-band application security testing (OAST) infrastructure from scratch. Post 1 hand-rolled a JNDI Reference; post 2 built the authoritative DNS server. This one is the riskiest feature in the whole platform: letting users run their own Python, in production, to shape the responses the listeners send back. The feature is simple to state and terrifying to implement: a user writes a Python function, and when a callback lands on their host, the server runs that function to decide what to answer. DNS rebinding, conditional payloads by source IP, crafting a JNDI reference on the fly — all of it wants user-supplied logic in the response path. Which means executing untrusted Python on your production box. ...