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

CVE ID : CVE-2026-27896

Published : Feb. 26, 2026, 12:47 a.m. | 21 minutes ago

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.

Severity: 0.0 | NA

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