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all: use case-sensitive JSON unmarshaling (#807)

Browse source 
As reported in
https://github.com/modelcontextprotocol/go-sdk/issues/805, case
insensitive unmarshaling can pose security issues. This change adjusts
all non-test and non-example unmarshaling to use a case sensitive mode.

This introduces a new external dependency
`github.com/segmentio/encoding`.
This commit is contained in:
Maciej Kisiel 2026-02-18 16:38:36 +01:00 • committed by GitHub
commit 7b8d81c264
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
16 changed files with 134 additions and 25 deletions
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6
go.mod
View file

@ -6,7 +6,13 @@ require (
github.com/golang-jwt/jwt/v5 v5.3.0
github.com/google/go-cmp v0.7.0
github.com/google/jsonschema-go v0.4.2
github.com/segmentio/encoding v0.5.3
github.com/yosida95/uritemplate/v3 v3.0.2
golang.org/x/oauth2 v0.34.0
golang.org/x/tools v0.41.0
)
require (
github.com/segmentio/asm v1.1.3 // indirect
golang.org/x/sys v0.40.0 // indirect
)

6
go.sum
View file

@ -4,9 +4,15 @@ github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
github.com/google/jsonschema-go v0.4.2 h1:tmrUohrwoLZZS/P3x7ex0WAVknEkBZM46iALbcqoRA8=
github.com/google/jsonschema-go v0.4.2/go.mod h1:r5quNTdLOYEz95Ru18zA0ydNbBuYoo9tgaYcxEYhJVE=
github.com/segmentio/asm v1.1.3 h1:WM03sfUOENvvKexOLp+pCqgb/WDjsi7EK8gIsICtzhc=
github.com/segmentio/asm v1.1.3/go.mod h1:Ld3L4ZXGNcSLRg4JBsZ3//1+f/TjYl0Mzen/DQy1EJg=
github.com/segmentio/encoding v0.5.3 h1:OjMgICtcSFuNvQCdwqMCv9Tg7lEOXGwm1J5RPQccx6w=
github.com/segmentio/encoding v0.5.3/go.mod h1:HS1ZKa3kSN32ZHVZ7ZLPLXWvOVIiZtyJnO1gPH1sKt0=
github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=
github.com/yosida95/uritemplate/v3 v3.0.2/go.mod h1:ILOh0sOhIJR3+L/8afwt/kE++YT040gmv5BQTMR2HP4=
golang.org/x/oauth2 v0.34.0 h1:hqK/t4AKgbqWkdkcAeI8XLmbK+4m4G5YeQRrmiotGlw=
golang.org/x/oauth2 v0.34.0/go.mod h1:lzm5WQJQwKZ3nwavOZ3IS5Aulzxi68dUSgRHujetwEA=
golang.org/x/sys v0.40.0 h1:DBZZqJ2Rkml6QMQsZywtnjnnGvHza6BTfYFWY9kjEWQ=
golang.org/x/sys v0.40.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
golang.org/x/tools v0.41.0 h1:a9b8iMweWG+S0OBnlU36rzLp20z1Rp10w+IY2czHTQc=
golang.org/x/tools v0.41.0/go.mod h1:XSY6eDqxVNiYgezAVqqCeihT4j1U2CCsqvH3WhQpnlg=

19
internal/json/json.go Normal file
View file

@ -0,0 +1,19 @@
// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
// Use of this source code is governed by the license
// that can be found in the LICENSE file.
// Package json provides internal JSON utilities.
package json
import (
"bytes"
"github.com/segmentio/encoding/json"
)
func Unmarshal(data []byte, v any) error {
dec := json.NewDecoder(bytes.NewReader(data))
dec.DontMatchCaseInsensitiveStructFields()
return dec.Decode(v)
}

63
internal/json/json_test.go Normal file
View file

@ -0,0 +1,63 @@
// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
// Use of this source code is governed by the license
// that can be found in the LICENSE file.
package json
import (
"testing"
"github.com/google/go-cmp/cmp"
)
func TestUnmarshalCaseSensitivity(t *testing.T) {
type Nested struct {
Field string `json:"field"`
}
type Target struct {
Field string
TaggedField string `json:"custom_tag"`
Nested *Nested
}
tests := []struct {
name string
json string
want Target
}{
{
name: "exact match",
json: `{"Field": "value", "custom_tag": "tagged", "Nested": {"field": "nested"}}`,
want: Target{
Field: "value",
TaggedField: "tagged",
Nested: &Nested{
Field: "nested",
},
},
},
{
name: "case mismatch",
json: `{"field": "value", "Custom_tag": "tagged", "Nested": {"Field": "nested"}}`,
want: Target{
Field: "",
TaggedField: "",
Nested: &Nested{
Field: "",
},
},
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
var got Target
if err := Unmarshal([]byte(tt.json), &got); err != nil {
t.Fatalf("Unmarshal failed: %v", err)
}
if diff := cmp.Diff(tt.want, got); diff != "" {
t.Errorf("Unmarshal mismatch (-want +got):\n%s", diff)
}
})
}
}

3
internal/jsonrpc2/conn.go
View file

@ -6,13 +6,14 @@ package jsonrpc2
import (
"context"
"encoding/json"
"errors"
"fmt"
"io"
"sync"
"sync/atomic"
"time"
"github.com/modelcontextprotocol/go-sdk/internal/json"
)
// Binder builds a connection configuration.

4
internal/jsonrpc2/messages.go
View file

@ -10,6 +10,8 @@ import (
"errors"
"fmt"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
)
@ -173,7 +175,7 @@ func EncodeIndent(msg Message, prefix, indent string) ([]byte, error) {
func DecodeMessage(data []byte) (Message, error) {
msg := wireCombined{}
if err := json.Unmarshal(data, &msg); err != nil {
if err := internaljson.Unmarshal(data, &msg); err != nil {
return nil, fmt.Errorf("unmarshaling jsonrpc message: %w", err)
}
if msg.VersionTag != wireVersion {

3
mcp/client.go
View file

@ -6,7 +6,6 @@ package mcp
import (
"context"
"encoding/json"
"errors"
"fmt"
"iter"
@ -18,7 +17,7 @@ import (
"time"
"github.com/google/jsonschema-go/jsonschema"
"github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
)

8
mcp/content.go
View file

@ -10,6 +10,8 @@ package mcp
import (
"encoding/json"
"fmt"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
)
// A Content is a [TextContent], [ImageContent], [AudioContent],
@ -248,7 +250,7 @@ func (c *ToolResultContent) MarshalJSON() ([]byte, error) {
return nil, err
}
var w wireContent
if err := json.Unmarshal(data, &w); err != nil {
if err := internaljson.Unmarshal(data, &w); err != nil {
return nil, err
}
contentWire = append(contentWire, &w)
@ -328,11 +330,11 @@ func unmarshalContent(raw json.RawMessage, allow map[string]bool) ([]Content, er
}
// Try array first, then fall back to single object.
var wires []*wireContent
if err := json.Unmarshal(raw, &wires); err == nil {
if err := internaljson.Unmarshal(raw, &wires); err == nil {
return contentsFromWire(wires, allow)
}
var wire wireContent
if err := json.Unmarshal(raw, &wire); err != nil {
if err := internaljson.Unmarshal(raw, &wire); err != nil {
return nil, err
}
c, err := contentFromWire(&wire, allow)

16
mcp/protocol.go
View file

@ -14,6 +14,8 @@ import (
"encoding/json"
"fmt"
"maps"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
)
// Optional annotations for the client. The client can use annotations to inform
@ -141,7 +143,7 @@ func (x *CallToolResult) UnmarshalJSON(data []byte) error {
res
Content []*wireContent `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
var err error
@ -315,7 +317,7 @@ type CompleteReference struct {
func (r *CompleteReference) UnmarshalJSON(data []byte) error {
type wireCompleteReference CompleteReference // for naive unmarshaling
var r2 wireCompleteReference
if err := json.Unmarshal(data, &r2); err != nil {
if err := internaljson.Unmarshal(data, &r2); err != nil {
return err
}
switch r2.Type {
@ -502,7 +504,7 @@ func (m *SamplingMessageV2) UnmarshalJSON(data []byte) error {
msg
Content json.RawMessage `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
var err error
@ -542,7 +544,7 @@ func (r *CreateMessageResult) UnmarshalJSON(data []byte) error {
result
Content *wireContent `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
var err error
@ -605,7 +607,7 @@ func (r *CreateMessageWithToolsResult) UnmarshalJSON(data []byte) error {
result
Content json.RawMessage `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
var err error
@ -1056,7 +1058,7 @@ func (m *PromptMessage) UnmarshalJSON(data []byte) error {
msg
Content *wireContent `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
var err error
@ -1253,7 +1255,7 @@ func (m *SamplingMessage) UnmarshalJSON(data []byte) error {
msg
Content *wireContent `json:"content"`
}
if err := json.Unmarshal(data, &wire); err != nil {
if err := internaljson.Unmarshal(data, &wire); err != nil {
return err
}
// Allow text, image, audio, tool_use, and tool_result in sampling messages

5
mcp/server.go
View file

@ -25,6 +25,7 @@ import (
"time"
"github.com/google/jsonschema-go/jsonschema"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/internal/util"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
@ -327,7 +328,7 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out], cache *SchemaCa
// Unmarshal and validate args.
var in In
if input != nil {
if err := json.Unmarshal(input, &in); err != nil {
if err := internaljson.Unmarshal(input, &in); err != nil {
return nil, fmt.Errorf("%w: %v", jsonrpc2.ErrInvalidParams, err)
}
}
@ -1371,7 +1372,7 @@ func initializeMethodInfo() methodInfo {
info.unmarshalParams = func(m json.RawMessage) (Params, error) {
var params *initializeParamsV2
if m != nil {
if err := json.Unmarshal(m, &params); err != nil {
if err := internaljson.Unmarshal(m, &params); err != nil {
return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, params, err)
}
}

3
mcp/shared.go
View file

@ -23,6 +23,7 @@ import (
"time"
"github.com/modelcontextprotocol/go-sdk/auth"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
)
@ -283,7 +284,7 @@ func newMethodInfo[P paramsPtr[T], R Result, T any](flags methodFlags) methodInf
unmarshalParams: func(m json.RawMessage) (Params, error) {
var p P
if m != nil {
if err := json.Unmarshal(m, &p); err != nil {
if err := internaljson.Unmarshal(m, &p); err != nil {
return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
}
}

3
mcp/streamable.go
View file

@ -30,6 +30,7 @@ import (
"time"
"github.com/modelcontextprotocol/go-sdk/auth"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
"github.com/modelcontextprotocol/go-sdk/internal/util"
@ -1105,7 +1106,7 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
isInitialize = true
// Extract the protocol version from InitializeParams.
var params InitializeParams
if err := json.Unmarshal(jreq.Params, &params); err == nil {
if err := internaljson.Unmarshal(jreq.Params, &params); err == nil {
initializeProtocolVersion = params.ProtocolVersion
}
}

3
mcp/tool.go
View file

@ -11,6 +11,7 @@ import (
"strings"
"github.com/google/jsonschema-go/jsonschema"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
)
// A ToolHandler handles a call to tools/call.
@ -83,7 +84,7 @@ func applySchema(data json.RawMessage, resolved *jsonschema.Resolved) (json.RawM
if resolved != nil {
v := make(map[string]any)
if len(data) > 0 {
if err := json.Unmarshal(data, &v); err != nil {
if err := internaljson.Unmarshal(data, &v); err != nil {
return nil, fmt.Errorf("unmarshaling arguments: %w", err)
}
}

5
mcp/transport.go
View file

@ -15,6 +15,7 @@ import (
"os"
"sync"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/internal/xcontext"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
@ -193,7 +194,7 @@ type canceller struct {
func (c *canceller) Preempt(ctx context.Context, req *jsonrpc.Request) (result any, err error) {
if req.Method == notificationCancelled {
var params CancelledParams
if err := json.Unmarshal(req.Params, &params); err != nil {
if err := internaljson.Unmarshal(req.Params, &params); err != nil {
return nil, err
}
id, err := jsonrpc2.MakeID(params.RequestID)
@ -569,7 +570,7 @@ func (t *ioConn) Read(ctx context.Context) (jsonrpc.Message, error) {
func readBatch(data []byte) (msgs []jsonrpc.Message, isBatch bool, _ error) {
// Try to read an array of messages first.
var rawBatch []json.RawMessage
if err := json.Unmarshal(data, &rawBatch); err == nil {
if err := internaljson.Unmarshal(data, &rawBatch); err == nil {
if len(rawBatch) == 0 {
return nil, true, fmt.Errorf("empty batch")
}

4
mcp/util.go
View file

@ -6,6 +6,8 @@ package mcp
import (
"encoding/json"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
)
func assert(cond bool, msg string) {
@ -21,7 +23,7 @@ func remarshal(from, to any) error {
if err != nil {
return err
}
if err := json.Unmarshal(data, to); err != nil {
if err := internaljson.Unmarshal(data, to); err != nil {
return err
}
return nil

8
oauthex/dcr.go
View file

@ -17,6 +17,8 @@ import (
"io"
"net/http"
"time"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
)
// ClientRegistrationMetadata represents the client metadata fields for the DCR POST request (RFC 7591).
@ -144,7 +146,7 @@ func (r *ClientRegistrationResponse) UnmarshalJSON(data []byte) error {
}{
alias: (*alias)(r),
}
if err := json.Unmarshal(data, &aux); err != nil {
if err := internaljson.Unmarshal(data, &aux); err != nil {
return err
}
if aux.ClientIDIssuedAt != 0 {
@ -206,7 +208,7 @@ func RegisterClient(ctx context.Context, registrationEndpoint string, clientMeta
if resp.StatusCode == http.StatusCreated {
var regResponse ClientRegistrationResponse
if err := json.Unmarshal(body, &regResponse); err != nil {
if err := internaljson.Unmarshal(body, &regResponse); err != nil {
return nil, fmt.Errorf("failed to decode successful registration response: %w (%s)", err, string(body))
}
if regResponse.ClientID == "" {
@ -221,7 +223,7 @@ func RegisterClient(ctx context.Context, registrationEndpoint string, clientMeta
if resp.StatusCode == http.StatusBadRequest {
var regError ClientRegistrationError
if err := json.Unmarshal(body, &regError); err != nil {
if err := internaljson.Unmarshal(body, &regError); err != nil {
return nil, fmt.Errorf("failed to decode registration error response: %w (%s)", err, string(body))
}
return nil, &regError