GHSA-wvj2-96wp-fq3f
Suggest an improvement- Source
- https://github.com/advisories/GHSA-wvj2-96wp-fq3f
- Import Source
- https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/02/GHSA-wvj2-96wp-fq3f/GHSA-wvj2-96wp-fq3f.json
- JSON Data
- https://api.test.osv.dev/v1/vulns/GHSA-wvj2-96wp-fq3f
- Aliases
-
- CVE-2026-27896
- Related
- Published
- 2026-02-26T22:20:08Z
- Modified
- 2026-02-28T03:29:06.452656Z
- Severity
-
- 7.0 (High) CVSS_V4 - 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 CVSS Calculator
- Summary
- MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity
- Details
-
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.
- Database specific
{ "nvd_published_at": "2026-02-26T01:16:25Z", "github_reviewed_at": "2026-02-26T22:20:08Z", "cwe_ids": [ "CWE-178", "CWE-436" ], "github_reviewed": true, "severity": "HIGH" }- References
Affected packages
Go / github.com/modelcontextprotocol/go-sdk
Package
- Name
- github.com/modelcontextprotocol/go-sdk
- View open source insights on deps.dev
- Purl
- pkg:golang/github.com/modelcontextprotocol/go-sdk