CVE-2026-27896

HIGH EPSS 18.2%
Published Feb 26, 20264mo ago · Modified Jun 17, 20262w ago
7.0 CVSS 4.0
High
Find Similar
Published Feb 26, 2026 4mo ago
Last Modified Jun 17, 2026 2w ago

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 Details

Base Score
7.0
Exploitability
—
Impact
—
Vector string
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Attack Vector Network
Attack Complexity Low
Privileges Required None
User Interaction None
Scope X

Threat Intelligence

EPSS Exploit Probability
18.2% percentile
Exploit & Patch Status
No Known Exploit
Patch Available

Weaknesses 2

CWE-178
CWE-436

Affected Products 1

VendorProductVersionRange
lfprojectsmcp_go_sdk* <1.3.1

References 2

  • github.com https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
    Patch
  • github.com https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
    PatchVendor Advisory

Remediation

  • github.com https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
    Patch
  • github.com https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
    PatchVendor Advisory