🛡️ CVE-2026-27896
🟠 CVSS 8.0 — High ✅ No Known Exploit CWE-178 NVD
8.0
CVSS Score
0 Low4 Medium7 High9 Critical10

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.

Details

Severity HIGH
CVSS Score 8.0
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
CWE CWE-178
Public Exploit ✅ No
Source NVD
Published 2026-02-26
Updated 2026-06-15
Modified 2026-03-23

Affected Packages

Software From version Fixed in
github.com/modelcontextprotocol/go-sdk — —
mcp-go-sdk — 1.3.1

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