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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.2 · 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 · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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…

Published by the source

19 Mar 2026, 12:44 UTC

Last modified by the source

19 Mar 2026, 12:44 UTC

Source last checked

26 Oct 2026, 15:18 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-q382-vc8q-7jhj

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# Patch Next advisory brief: GHSA-q382-vc8q-7jhj

Advisory: Improper handling of null Unicode character when parsing JSON in github.com/modelcontextprotocol/go-sdk
IDs: GHSA-q382-vc8q-7jhj
Record: https://patchnext.com/advisories/ghsa-q382-vc8q-7jhj (revision 1, source checked 26 Oct 2026, 15:18 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/modelcontextprotocol/go-sdk 1.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/modelcontextprotocol/go-sdk: affected <= 1.4.0; fixed in 1.4.1

## Suggested inspection (read-only)
- Go github.com/modelcontextprotocol/go-sdk: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/modelcontextprotocol/go-sdk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-q382-vc8q-7jhj (GitHub-reviewed advisory GHSA-q382-vc8q-7jhj lists affected ranges: Go github.com/modelcontextprotocol/go-sdk >=0 <=1.4.0)

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