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

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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.

Source published

19 Mar 2026 12:44:07 UTC

Source modified

10 Sep 2026 03:50:40 UTC

Saved here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:L/SA:N

Publisher rating · HIGH

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

Source published

23 Mar 2026 18:14:51 UTC

Source modified

25 Mar 2026 15:29:13 UTC

Saved here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · OSV · upstream source license applies. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.