Case Sensitivity Issue in Go MCP SDK by Model Context Protocol
CVE-2026-27896

7HIGH

Key Information:

Vendor

Modelcontextprotocol

Status
Go-sdk
Vendor
CVE Published:
26 February 2026

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.

Affected Version(s)

go-sdk < 1.3.1

References

https://github.com/modelcontextprotocol/go-sdk/security/a...
x_refsource_CONFIRM
https://github.com/modelcontextprotocol/go-sdk/commit/7b8...
x_refsource_MISC

CVSS V4

Score:
7
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
Physical
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.