Improper handling of null Unicode character when parsing JSON in github.com/modelcontextprotocol/go-sdk

The Go SDK recently transitioned to the segmentio/encoding library for JSON parsing in version 1.3.1. While this change addressed both case-insensitivity and ASCII folding issues, the new parser implemented aggressive key matching that treated keys with null Unicode characters appended at the end as equivalent to their base strings.

Impact

When combined with duplicate keys, the described behavior leads to a "last key wins" resolution that could override the intended MCP message. This had the potential for:

• Bypassing intermediary inspection: Proxies or policy layers that matched on exact field names may have failed to detect or filter these messages.
• Cross-implementation inconsistency: Other MCP SDKs (TypeScript, Python) use case-sensitive parsing and would reject the same messages, creating potential security-boundary confusion.

Fix:

The segmentio/encoding package was patched with a fix in https://github.com/segmentio/encoding/commit/7d5a25dbc5da13aed3cb047a127e4d0e96f536fb and a new version of the package was released (v0.5.4). The SDK switched to the patched version of the dependency in 724dd47aa. Users are advised to update to v1.4.1 to resolve this issue.

Credits:

Thank you to Francesco Lacerenza (Doyensec) for reporting this issue.