Vulnerability Details CVE-2026-27896
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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.0
EPSS Ranking 13.2%
CVSS Severity
CVSS v3 Score 7.5
References
Products affected by CVE-2026-27896
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cpe:2.3:a:lfprojects:mcp_go_sdk:*