---
id: CVE-2026-27896
title: MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity
summary: >-
  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
  tagg…
severity: high
cvss: 7
cvssVector: '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'
cvssSource: cna
cwe:
  - CWE-178
  - CWE-436
vendor: modelcontextprotocol
product: go-sdk
affected:
  - go-sdk < 1.3.1
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-02-26T17:06:14.973622Z'
published: '2026-02-26'
updated: '2026-09-16'
sourceUpdated: '2026-09-16T12:04:21.685Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-27896'
references:
  - url: >-
      https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
    label: >-
      https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
  - url: >-
      https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
    label: >-
      https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
tags:
  - cve.org
epss: 0.0044
epssPercentile: 0.35495
ingestedAt: '2026-09-16T12:55:21.866Z'
---

## Overview

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

- `go-sdk < 1.3.1`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.