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

Advisory record · `GHSA-q382-vc8q-7jhj`

- **Identifiers:** `GHSA-q382-vc8q-7jhj` · `GO-2026-4770`
- **State:** Confirmed
- **Severity:** CVSS 4.0, no score published
- **Critical:** No
- **Weakness (CWE):** [CWE-436](https://cwe.mitre.org/data/definitions/436.html) · [CWE-1395](https://cwe.mitre.org/data/definitions/1395.html)
- **Tags:** [CWE-436](https://agentprime.io/advisories/tag/cwe-436.md) · [CWE-1395](https://agentprime.io/advisories/tag/cwe-1395.md)
- **Published:** 19 Mar 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 19 Mar 2026, 12:44 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `github.com/modelcontextprotocol/go-sdk` (Go): affected `<1.4.1`; fixed in `1.4.1`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> The Go SDK github.com/modelcontextprotocol/go-sdk versions before 1.4.1 are affected. Its JSON parsing treated keys with null Unicode characters appended to them as equal to the base key, so when duplicate keys were present the last one could override the intended message. An attacker able to send crafted MCP messages could potentially evade proxies or policy layers that match exact field names. Upgrade to version 1.4.1 or later.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- Score not published, CVSS 4.0: `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`, from GitHub Advisory Database

- **CISA KEV:** Not listed.
- **EPSS:** No score.

## Timeline

- 31 Oct 2026, 14:14 UTC: First seen by agentprime
- 31 Oct 2026, 14:14 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-q382-vc8q-7jhj)

## Sources

- [GitHub Advisory Database: GHSA-q382-vc8q-7jhj](https://github.com/advisories/GHSA-q382-vc8q-7jhj)
- [OSV: GHSA-q382-vc8q-7jhj](https://osv.dev/vulnerability/GHSA-q382-vc8q-7jhj)
- <https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-q382-vc8q-7jhj>
- <https://github.com/modelcontextprotocol/go-sdk/commit/724dd47aa3431b9d4cf9ac2eebbf7b38a629afca>
- <https://github.com/segmentio/encoding/commit/7d5a25dbc5da13aed3cb047a127e4d0e96f536fb>
- <https://github.com/modelcontextprotocol/go-sdk>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-q382-vc8q-7jhj>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-q382-vc8q-7jhj/>. Everything for agents: <https://agentprime.io/llms.txt>.