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name: hunt-exceptional-conditions description: "Hunt mishandling of exceptional conditions — feed an endpoint malformed/unexpected input (wrong type, broken JSON, oversized field, null byte) and make it fail OPEN or leak internals: a verbose stack-trace / framework error page that discloses ORM internals, server file paths, library versions, or a language traceback. Use on any input-accepting endpoint (JSON APIs, forms, query params). Medium-High when the leak exposes internal structure that arms a deeper attack." report_count: 0 sources: hackerone_public

HUNT-EXCEPTIONAL-CONDITIONS — Verbose Errors / Fail-Open (A10:2025)

What actually pays

Well-built apps catch errors and return a clean, generic message. A broken app, when handed input it didn't expect, throws an unhandled exception and renders a developer error page straight to the client — leaking the stack trace, the ORM/query internals, server-side file paths, and framework/library versions. That disclosure is the finding (and it arms SQLi/RCE/path attacks next).

Recon

Any endpoint that parses input is a candidate; the richest are:

JSON APIs that expect typed fields:  POST /api/* with {numbers, ids, enums}
Endpoints with numeric/id path or query params:  /item/{id}, ?page=, ?quantity=
Search / filter / sort params
File or content-type sensitive uploads

Attack — send what the code didn't anticipate

Take a known-good request and break ONE assumption at a time:

• Wrong type: a field the app expects to be a number/string is sent as an array or object — {"rating":"x","comment":[1,2,3]}, {"quantity":{}}.
• Malformed body: truncated/!invalid JSON, an unterminated string, a stray brace, a wrong/missing Content-Type.
• Boundary/oversized: a very long string, a huge/negative/overflow number.
• Null byte / control chars embedded in a value.

POST /api/Feedbacks   {"rating":"notanumber","comment":[1,2,3]}
GET  /item/' OR /item/%00   (also exercises the error path)

Watch the RESPONSE BODY, not just the status: a 500 (or even a 200/400) whose body contains a stack trace or framework error page is the signal.

What counts as a leak (the success signal)

A finding is confirmed when the response body contains a cross-framework error-disclosure signature:

• Node/Express + Sequelize: SequelizeDatabaseError, node_modules/sequelize, a JS stack with internal paths.
• PHP: <b>Warning</b> ... /var/www/.../file.php on line N.
• Python: Traceback (most recent call last), werkzeug.exceptions.
• Java: at com.app.Foo(Foo.java:42) stack frames.
• .NET: Server Error in '/' Application, a [System.XxxException: ...] YSOD.

A clean JSON error ({"error":"Invalid input"}) with no internals is NOT a finding — that's correct handling. Disclosure of internal structure is.

Validation discipline

• Capture the exact leaked artifact (path, ORM class, version, stack frame) — that's the evidence. "It returned 500" alone is not disclosure.
• Note what the leak enables next (e.g. a disclosed SQL error → hunt-sqli; a disclosed absolute path → hunt-lfi).