Upstream information

CVE-2026-27896 at MITRE

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.

SUSE information

Overall state of this security issue: Resolved

This issue is currently rated as having important severity.

CVSS v4 Scores
CVSS detail CNA (GitHub)
Base Score 7
Vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User Interaction None
Vulnerable System Confidentiality Impact None
Vulnerable System Integrity Impact None
Vulnerable System Availability Impact None
Subsequent System Confidentiality Impact High
Subsequent System Integrity Impact High
Subsequent System Availability Impact None
CVSSv4 Version 4.0
No SUSE Bugzilla entries cross referenced.

SUSE Security Advisories:

List of released packages

Product(s) Fixed package version(s) References

SUSE Timeline for this CVE

CVE page created: Thu Feb 26 04:02:50 2026
CVE page last modified: Thu Mar 26 20:49:53 2026