defensive-programming

Defensive Programming at Trust Boundaries

The discipline of defending code where it actually matters — system boundaries — without smothering internal code in redundant checks. Over- defending is its own bug: it hides real failures, balloons code for no safety gain, and makes intent harder to read.

Steps

1. Identify the actual trust boundaries first. Defense belongs at the edges where untrusted data enters: user input, external API responses, file/network/DB reads, deserialized payloads, environment/config. Code that only ever receives data already validated by an internal caller does not need to re-validate — trust your own contracts.

2. At each boundary, check for the OWASP-class basics relevant to it:

   • Injection (SQL/command/template/log) — use parameterized queries, never string-concatenate untrusted input into commands or queries
   • Deserialization — validate shape/type before trusting structure; never eval/deserialize untrusted data into executable form
   • Auth/access — verify identity and authorization at the boundary, not just one or the other
   • Resource exhaustion — bound input size, recursion depth, request rates where the boundary is externally reachable
   • Secrets — never log, echo, or include credentials/tokens in error messages or traces that might reach untrusted eyes

3. Fail safely and legibly:

   • Reject invalid input with a clear error at the boundary — don't let malformed data silently propagate inward and fail confusingly later
   • Don't swallow exceptions into empty catch blocks — that converts a loud, diagnosable failure into a silent, undebuggable one
   • Error messages to untrusted callers should not leak internals (stack traces, query structure, file paths); detailed diagnostics belong in logs

4. Don't defend against the impossible. Validating internal function arguments that the type system or an internal contract already guarantees is noise — it adds code, obscures the real boundary checks, and signals "I don't trust my own code," which erodes the value of having contracts at all. If you find yourself validating the same thing at every layer, that's a sign the check belongs at the boundary, once.