Unsafe API Consumption Check (API10:2023)

What this checks

Protects against blindly trusting data from third-party APIs. External API responses can be tampered with (via MITM, compromised provider, or supply-chain attack), contain unexpected types or malicious payloads, or change without notice. Treating external data as trusted leads to injection, deserialization attacks, and business logic bypass.

Vulnerable patterns

• External API response fields flowing directly into SQL, shell, template, or code-execution sinks via string interpolation
• Rendering third-party HTML or markdown without a sanitization step
• Merging unvalidated external response objects into internal models
• Following redirect URLs returned by an external API without re-validating against the same allowlist
• Calling external APIs without size limits or timeouts, allowing a partner to stream gigabytes or hang the caller

Fix immediately

Flag the vulnerable code, explain the risk, and suggest a fix establishing these properties. Translate to the HTTP client, schema library, and template engine of the audited file — use that stack's documented validation, sanitization, and redirect controls; do not import a recipe from a different stack.

1. Every response is validated against an explicit schema — typed struct, schema validator, JSON Schema, protobuf message, or equivalent — before any field reaches business logic. Optional fields get defaults; unexpected fields are rejected or ignored, not silently propagated.
2. External data never reaches injection sinks via string interpolation. SQL goes through parameterized queries; HTML goes through an auto-escaping template engine; shell goes through argv arrays. Validation does not exempt the sink.
3. Response size is bounded and the request has a timeout. A partner API can stream gigabytes or hang forever; both eat resources and can DoS the caller.
4. Redirects are not followed blindly. Either disable redirects, or re-validate each hop against the same allowlist used for the initial URL. Partner-controlled 3xx responses can redirect to SSRF targets or attacker-controlled hosts.

Verification

• Every response from an external API is validated against a schema before any field is used in queries, rendering, or business logic
• External API data never reaches SQL, shell, template, or code-execution sinks via string interpolation — only through parameterized/auto-escaping interfaces
• HTTP responses from external APIs are size-limited and the request has a timeout
• Redirects from external API responses are either disabled or re-validated at each hop against the same allowlist used for the initial URL

References

• CWE-20 (Improper Input Validation)
• CWE-502 (Deserialization of Untrusted Data)
• OWASP API10:2023 Unsafe Consumption of APIs