MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-27896: 0.29% estimated exploitation activity in the next 30 days; percentile 21.86 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing. 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. Additionally, Go's standard library folds the Unicode characters ſ (U+017F) and K (U+212A) to their ASCII equivalents s and k, meaning fields like "paramſ" would match "params". This violated the JSON-RPC 2.0 specification, which defines exact field names. Impact: A malicious MCP peer may have been able to send protocol messages with non-standard field casing (e.g., "Method" instead of "method") that the SDK would silently accept. 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…

Published by the source

26 Feb 2026, 22:20 UTC

Last modified by the source

26 Feb 2026, 22:20 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-wvj2-96wp-fq3f

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-27896

Advisory: MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity
IDs: CVE-2026-27896, GHSA-wvj2-96wp-fq3f
Record: https://patchnext.com/advisories/cve-2026-27896 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.0)
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 1.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/modelcontextprotocol/go-sdk: affected < 1.3.1; fixed in 1.3.1

## 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-wvj2-96wp-fq3f (GitHub-reviewed advisory GHSA-wvj2-96wp-fq3f lists affected ranges: Go github.com/modelcontextprotocol/go-sdk >=0 <1.3.1)

Commands are suggestions: review each one before running it.