CVE-2026-27896

HighCVSS 7.5/10Last modified Jul 15, 2026

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.

Metrics

CVSS 3.1 7.5/10CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Weakness enumeration

CWE-178 CWE-436

Affected software

Vendor	Product	Versions
modelcontextprotocol	go-sdk	< 1.3.1
Red Hat	Migration Toolkit for Virtualization	—
Red Hat	Migration Toolkit for Virtualization	—
Red Hat	Migration Toolkit for Virtualization	—
Red Hat	Migration Toolkit for Virtualization	—
Red Hat	Migration Toolkit for Virtualization	—
Red Hat	OpenShift Lightspeed	—
Red Hat	OpenShift Serverless	—
Red Hat	OpenShift Serverless	—
Red Hat	Red Hat OpenShift AI (RHOAI)	—
Red Hat	Red Hat OpenShift AI (RHOAI)	—
Red Hat	Red Hat OpenShift AI (RHOAI)	—

References

Timeline

Published: Feb 26, 2026
Last modified: Jul 15, 2026
NVD status: Modified

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