CVE-2026-27896

Summary

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.

Affected Software

Weaknesses

• CWE-178: CWE-178: Improper Handling of Case Sensitivity
• CWE-436: CWE-436: Interpretation Conflict

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: none
  • Automatable: yes
  • Technical Impact: partial

modelcontextprotocol/go-sdk: improper handling of case sensitivity

Additional References

• https://access.redhat.com/security/cve/CVE-2026-27896
• https://bugzilla.redhat.com/show_bug.cgi?id=2442903
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json

References

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

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