CVE-2026-100352

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.148.0, the OpenAPI 3.1 validation path passes YAML NaN and infinity numeric values to santhosh-tekuri/jsonschema v6.0.2 without applying kin-openapi's existing finite-number guards. A remote unauthenticated client can send an application/yaml or application/x-yaml request body containing .nan, .inf, or -.inf to an operation with a numeric constraint, causing the dependency to create and dereference a nil big.Rat during ValidateRequest. Standard net/http servers recover the panic per connection, while other hosts or application goroutines may terminate the process. This issue is fixed in version 0.148.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.148.0, the OpenAPI 3.1 validation path passes YAML NaN and infinity numeric values to santhosh-tekuri/jsonschema v6.0.2 without applying kin-openapi's existing finite-number guards. A remote unauthenticated client can send an application/yaml or application/x-yaml request body containing .nan, .inf, or -.inf to an operation with a numeric constraint, causing the dependency to create and dereference a nil big.Rat during ValidateRequest. Standard net/http servers recover the panic per connection, while other hosts or application goroutines may terminate the process. This issue is fixed in version 0.148.0.

Summary

kin-openapi guards against NaN and Inf values in its built-in schema visitor (visitJSON) by returning ErrSchemaInputNaN / ErrSchemaInputInf when a float64 input violates those constraints. For OAS 3.1 specs, however, Schema.VisitJSON routes through useJSONSchema2020, which hands the value directly to santhosh-tekuri/jsonschema/v6 v6.0.2 without applying the same guard.

Inside jsonschema/v6 v6.0.2, numValidate builds a *big.Rat from the input via big.Rat.SetString(fmt.Sprintf("%v", v)). For NaN/Inf this parse fails silently and returns nil. numValidate then dereferences the nil *big.Rat (in math/big.(*Rat).Cmp), producing a Go runtime panic.

An attacker delivers this by sending an HTTP request with Content-Type: application/yaml (or application/x-yaml) and a YAML body containing .nan, .inf, or -.inf. The YamlBodyDecoder decodes those tokens into math.NaN() / math.Inf(±1) (standard YAML 1.2 behavior), and they propagate to the unguarded useJSONSchema2020 path whenever the schema declares a numeric constraint (minimum, maximum, exclusiveMinimum, exclusiveMaximum, or multipleOf).

Details

Root Cause

The built-in visitor visitJSON at openapi3/schema.go:1968 guards NaN/Inf explicitly:

case float64:
    if math.IsNaN(value) {
        return ErrSchemaInputNaN
    }
    if math.IsInf(value, 0) {
        return ErrSchemaInputInf
    }

For OAS 3.1 specs, VisitJSON dispatches to useJSONSchema2020 (openapi3/schema_jsonschema_validator.go:204), which contains no such guard:

func (schema *Schema) useJSONSchema2020(settings *schemaValidationSettings, value any) error {
    validator, err := newJSONSchemaValidator(schema)
    if err != nil {
        return schema.visitJSON(settings, value)
    }
    return validator.validate(value)      // value handed straight through
}

jsonSchemaValidator.validate calls jsonschema/v6.Schema.Validate (line 146), which reaches numValidate in v6.0.2. There, big.Rat.SetString(fmt.Sprintf("%v", v)) returns (nil, false) for NaN/Inf, and the code dereferences the nil *big.Rat in Rat.Cmp, causing the panic.

Proof of Concept

Environment

• kin-openapi v0.144.0 (tag v0.144.0, commit f0407d5)
• santhosh-tekuri/jsonschema/v6 v6.0.2 (transitive dep)
• Go 1.26.7

Spec

Minimal OAS 3.1 spec (spec.yaml) declaring a YAML body with a numeric constraint:

openapi: '3.1.0'
info: { title: Test, version: '1.0' }
paths:
  /test:
    post:
      requestBody:
        required: true
        content:
          application/yaml:
            schema:
              type: number
              minimum: 0
      responses:
        '200': { description: OK }

Trigger

A single POST /test with Content-Type: application/yaml and body .nan, .inf, or -.inf. All three variants panic:

=== RUN   TestNaNInf
[nan]    PANIC (confirms vuln): runtime error: invalid memory address or nil pointer dereference
[inf]    PANIC (confirms vuln): runtime error: invalid memory address or nil pointer dereference
[neginf] PANIC (confirms vuln): runtime error: invalid memory address or nil pointer dereference

Panic stack trace (confirms exact chain)

panic: runtime error: invalid memory address or nil pointer dereference

math/big.(*Rat).Cmp(0x0, ...)                                       <- nil *big.Rat dereferenced
    C:/Program Files/Go/src/math/big/rat.go:492
github.com/santhosh-tekuri/jsonschema/v6.(*validator).numValidate(...)
    .../jsonschema/v6@v6.0.2/validator.go:525
github.com/santhosh-tekuri/jsonschema/v6.(*validator).validate(...)
    .../jsonschema/v6@v6.0.2/validator.go:168
github.com/santhosh-tekuri/jsonschema/v6.(*Schema).validate(...)
    .../jsonschema/v6@v6.0.2/validator.go:34
github.com/getkin/kin-openapi/openapi3.(*jsonSchemaValidator).validate(...)
    openapi3/schema_jsonschema_validator.go:146…

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.148.0, the OpenAPI 3.1 validation path passes YAML NaN and infinity numeric values to santhosh-tekuri/jsonschema v6.0.2 without applying kin-openapi's existing finite-number guards. A remote unauthenticated client can send an application/yaml or application/x-yaml request body containing .nan, .inf, or -.inf to an operation with a numeric constraint, causing the dependency to create and dereference a nil big.Rat during ValidateRequest. Standard net/http servers recover the panic per connection, while other hosts or application goroutines may terminate the process. This issue is fixed in version 0.148.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.148.0, the OpenAPI 3.1 validation path passes YAML NaN and infinity numeric values to santhosh-tekuri/jsonschema v6.0.2 without applying kin-openapi's existing finite-number guards. A remote unauthenticated client can send an application/yaml or application/x-yaml request body containing .nan, .inf, or -.inf to an operation with a numeric constraint, causing the dependency to create and dereference a nil big.Rat during ValidateRequest. Standard net/http servers recover the panic per connection, while other hosts or application goroutines may terminate the process. This issue is fixed in version 0.148.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.148.0, the OpenAPI 3.1 validation path passes YAML NaN and infinity numeric values to santhosh-tekuri/jsonschema v6.0.2 without applying kin-openapi's existing finite-number guards. A remote unauthenticated client can send an application/yaml or application/x-yaml request body containing .nan, .inf, or -.inf to an operation with a numeric constraint, causing the dependency to create and dereference a nil big.Rat during ValidateRequest. Standard net/http servers recover the panic per connection, while other hosts or application goroutines may terminate the process. This issue is fixed in version 0.148.0.