{"id":"CVE-2026-27896","aliases":["GHSA-wvj2-96wp-fq3f","GO-2026-4569"],"url":"https://o3.security/vulnerability/CVE-2026-27896","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 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.","published":"2026-02-26T00:47:46.967Z","modified":"2026-07-31T18:31:06.063085324Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/modelcontextprotocol/go-sdk","fixedVersion":"1.3.1"}],"fix":{"url":"https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0","label":"modelcontextprotocol/go-sdk@7b8d81c"},"references":[{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-27896"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27896.json"},{"type":"ADVISORY","url":"https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27896"},{"type":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2442903"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-31T18:31:06.063085324Z"}}