CVE-2026-27896
MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity
- State
- Published
- Severity
- 7.0 High
- Published
- Feb 26, 2026
- Updated
- Sep 16, 2026
- Reserved
- Feb 24, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 7.0 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 | GitHub_M |
| 3.1 | 7.2 High | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N | redhat-SADP |
Weaknesses
Affected Products
| Product | Versions |
|---|---|
go-sdkmodelcontextprotocol |
|
References
- https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f
- https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0
- https://access.redhat.com/security/cve/CVE-2026-27896vdb-entry
- RHBZ#2442903issue-tracking
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json
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