CVE-2026-27896: Lfprojects Mcp Go SDK

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Lfprojects Mcp Go SDK: before 1.3.1 (fixed in 1.3.1)

Published 2026-02-26. Last modified 2026-07-15.