What’s Vulnerable › Vulnerabilities

CVE-2026-27896: MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-02-26 · updated 2026-07-15
• Weakness: CWE-178

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.

Affected products

• modelcontextprotocol go-sdk: before 1.3.1 — fixed in 1.3.1

How to fix

Upgrade to a fixed version: modelcontextprotocol go-sdk 1.3.1. Check the vendor advisory for mitigations.

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

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