要点
- CVE-2026-27896 是影响 github.com/modelcontextprotocol/go-sdk, mcp go sdk (lfprojects) 的高危 (CVSS 7.5)漏洞。
- 未来 30 天内被利用的概率（EPSS）：0.29%，高于 21.8% 的漏洞。
- 修复：升级到 1.3.1 或更高版本。
Description
The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing. 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. Additionally, Go's standard library folds the Unicode characters ſ (U+017F) and K (U+212A) to their ASCII equivalents s and k, meaning fields like "paramſ" would match "params". This violated the JSON-RPC 2.0 specification, which defines exact field names.
Impact:
A malicious MCP peer may have been able to send protocol messages with non-standard field casing (e.g., "Method" instead of "method") that the SDK would silently accept. This had the potential for:
- Bypassing intermediary inspection: Proxies or policy layers that matched on exact field names may have failed to detect or filter these messages.
- Cross-implementation inconsistency: Other MCP SDKs (TypeScript, Python) use case-sensitive parsing and would reject the same messages, creating potential security-boundary confusion.
Fix:
Go's standard JSON unmarshaling was replaced with a case-sensitive decoder (github.com/segmentio/encoding) in commit 7b8d81c. Users are advised to update to v1.3.1 to resolve this issue.
Credits:
MCP Go SDK thanks Francesco Lacerenza (Doyensec) for reporting this issue.
NVD 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.
github.com/modelcontextprotocol/go-sdk 的其他漏洞
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