Description (CNA)
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 (CNA)
| Vendor / product | Versions |
|---|---|
| modelcontextprotocol / go-sdk | affected < 1.3.1 |
CWE
CWE-178, CWE-436
References
- https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
- https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
ADP enrichment
No English description supplied.
No references supplied.
A flaw was found in the Go MCP SDK. This issue occurs due to an improper handling of case sensitivity during JSON-RPC message parsing, specifically in the matching of JSON keys to struct field tags. This behavior violates the JSON-RPC 2.0 specification, which explicitly requires case-sensitive field name matching. A malicious MCP peer able to send protocol messages with non-standard field casing can potentially bypass intermediary inspection, allowing attackers to smuggle payloads past upstream filters and cause cross-implementation inconsistency.