CVE-2026-27896: lfprojects mcp_go_sdk Improper Handling of Case Sensitivity

Vulnerability summary

CVE-2026-27896 is a high (CVSS 7.5) Improper Handling of Case Sensitivity vulnerability in lfprojects mcp_go_sdk. EPSS estimates a 0.47% chance of exploitation in the next 30 days, higher than 39% of scored vulnerabilities.

Timeline

1. 26-02-2026
   PublishedVulnerability first disclosed.
2. 16-09-2026
   Last modifiedVulnerability information updated.

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.

• CWE-178Improper Handling of Case SensitivityThe product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.
• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.