{"cve":"CVE-2026-27896","nvd":{"published":"2026-02-26T01:16:25.630","modified":"2026-07-15T02:19:12.600","status":"Modified","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_v4":{"vector":"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","score":7.0},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N","score":7.5},"cwes":["CWE-178","CWE-436"],"references":["https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0","https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f","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"],"products":["lfprojects:mcp_go_sdk"],"source":"nvd","synced_at":"2026-10-19T22:24:17.937441719+00:00"},"epss":{"cve":"CVE-2026-27896","score":0.00292,"percentile":0.21826,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-wvj2-96wp-fq3f","url":"https://osv.dev/vulnerability/GHSA-wvj2-96wp-fq3f","summary":"MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity","modified":"2026-09-10T03:50:36.139524509Z","packages":[{"ecosystem":"Go","name":"github.com/modelcontextprotocol/go-sdk","purl":"pkg:golang/github.com/modelcontextprotocol/go-sdk","ranges":["<1.3.1"],"fixed":["1.3.1"]}]},{"id":"GO-2026-4569","url":"https://osv.dev/vulnerability/GO-2026-4569","summary":"MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity in github.com/modelcontextprotocol/go-sdk","modified":"2026-03-23T04:52:45.041522Z","packages":[{"ecosystem":"Go","name":"github.com/modelcontextprotocol/go-sdk","purl":"pkg:golang/github.com/modelcontextprotocol/go-sdk","ranges":["<1.3.1"],"fixed":["1.3.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}