Vulnerability 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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Lfprojects | Mcp Go Sdk | < 1.3.1 |
Related Weaknesses (CWE)
References
- https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16ePatch
- https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wPatchVendor Advisory
FAQ
What is CVE-2026-27896?
CVE-2026-27896 is a vulnerability with a CVSS score of 7.5 (HIGH). 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 key...
How severe is CVE-2026-27896?
CVE-2026-27896 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-27896?
Check the references section above for vendor advisories and patch information. Affected products include: Lfprojects Mcp Go Sdk.