Case Sensitivity Issue in Go MCP SDK by Model Context Protocol

CVE-2026-27896

What is CVE-2026-27896?

The Go MCP SDK prior to version 1.3.1 exhibited a significant vulnerability stemming from its reliance on Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing, which permits case-insensitive matching of JSON keys. This implementation flaw posed a risk of accepting protocol messages with non-standard field casings, potentially allowing malicious entities to bypass protocol validation checks. This behavior conflicted with the JSON-RPC 2.0 specification that mandates precise field naming. To address this vulnerability, an update to version 1.3.1 introduced a case-sensitive decoder, ensuring compliance and enhanced security against intermediary inspection evasion.

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