CVE-2026-76905

### Summary A nil-pointer dereference in `openapi3filter.ConvertErrors` lets any unauthenticated client crash a server with a single HTTP request. When an application validates a `multipart/form-data` request body and renders the resulting validation error through the library-provided `ValidationErrorEncoder` / `ConvertErrors` helpers, a malformed scalar form field (e.g. a non-numeric value for an `integer` property) produces an error shape that `convertParseError` dereferences without a nil check. The handler goroutine panics, causing a denial of service. `application/json` request bodies are **not** affected — the bug is specific to `multipart/form-data`. ### Details The panic is in `convertParseError`, at [`openapi3filter/validation_error_encoder.go:119-120`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/validation_error_encoder.go#L119) (still present on `master` at the time of writing): ```go } else if innerErr.RootCause() != nil { if rootErr, ok := innerErr.Cause.(*ParseError); ok && rootErr.Kind == KindInvalidFormat && e.Parameter.In == "query" { // ❌ e.Parameter may be nil → panic ``` The comparison `e.Parameter.In == "query"` assumes `e.Parameter` is non-nil. It is reached whenever **both** of the following hold: 1. **`e.Parameter == nil`.** A `*RequestError` carries *either* `Parameter` (parameter errors) *or* `RequestBody` (body errors), never both. `ValidateRequestBody` builds body errors with only `RequestBody` set, leaving `Parameter` nil — see [`validate_request.go:326-332`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/validate_request.go#L326). 2. **`innerErr.Cause` is itself a `*ParseError`** (a `ParseError` nested inside a `ParseError`), so the type assertion on line 119 succeeds and execution reaches the `e.Parameter.In` dereference on line 120. The only default code path that satisfies *both* conditions is the **multipart** body decoder, which wraps a failed part's `*ParseError` inside a