CVE-2026-27896
Description
The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing in versions prior to 1.3.1. Go's standard library performs case-insensitive matching of JSON keys to struct field tags — a field tagged json:"method" would also match "Method", "METHOD", etc. This violated the JSON-RPC 2.0 specification, which defines exact field names. A malicious MCP peer may have been able to send protocol messages with non-standard field casing that the SDK would silently accept. This had the potential for bypassing intermediary inspection and coss-implementation inconsistency. Go's standard JSON unmarshaling was replaced with a case-sensitive decoder in commit 7b8d81c. Users are advised to update to v1.3.1 to resolve this issue.
How to fix
- lfprojects mcp_go_sdk1.3.1 or later
- github.com/modelcontextprotocol/go-sdk1.3.1 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| lfprojects mcp_go_sdk | v < 1.3.1 | 1.3.1 |
| github.com/modelcontextprotocol/go-sdkGo | v < 1.3.1 | 1.3.1 |
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