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Vulnerability · Published Feb 26, 2026 · Assigned by GitHub_M

CVE-2026-27896MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

High 7.2

In plain English

A high-severity security flaw in go-sdk from modelcontextprotocol. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.2High
Exploit odds (30 days)0.3%top 78.1% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Gogithub.com/modelcontextprotocol/go-sdk1.3.1GHSA-wvj2-96wp-fq3f
Gogithub.com/modelcontextprotocol/go-sdk1.3.1GO-2026-4569

Affected products (as reported)

VendorProductVersions
modelcontextprotocol go-sdk < 1.3.1

Weakness: CWE-178, CWE-436 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

References