CVE-2026-27896 (GCVE-0-2026-27896)

Vulnerability from cvelistv5 – Published: 2026-02-26 00:47 – Updated: 2026-02-26 17:06
VLAI?
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 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 ?
7 (High)
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
CWE
  • CWE-178 - Improper Handling of Case Sensitivity
  • CWE-436 - Interpretation Conflict
Assigner
References
Impacted products
Vendor Product Version
modelcontextprotocol go-sdk Affected: < 1.3.1
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