# CVE-2026-27896: MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

> A malicious peer can bypass intermediary inspection by sending JSON-RPC messages with non-standard field casing due to case-insensitive parsing in the Go MCP SDK.

- **Severity:** High 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Feb 2026
- **Updated:** 16 Sep 2026
- **Also known as:** CLEANSTART-2026-DZ42058, CLEANSTART-2026-EM35887, GHSA-wvj2-96wp-fq3f, GO-2026-4569, MINI-ccx8-75xj-wcv6, MINI-r9gj-5wqm-qpq4

## What is CVE-2026-27896?

CVE-2026-27896 is a high-severity vulnerability in modelcontextprotocol go-sdk, published on 26 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing in versions prior to 1.3.1. 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. This violated the JSON-RPC 2.0 specification, which defines exact field names. A malicious MCP peer may have been able to send protocol messages with non-standard field casing that the SDK would silently accept. This had the potential for bypassing intermediary inspection and coss-implementation inconsistency. Go's standard JSON unmarshaling was replaced with a case-sensitive decoder in commit 7b8d81c. Users are advised to update to v1.3.1 to resolve this issue.

Weakness: CWE-436 (Interpretation Conflict), CWE-178 (Improper Handling of Case Sensitivity)

## Which versions are affected by CVE-2026-27896?

3 open source packages ship an affected version, including github.com/modelcontextprotocol/go-sdk. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/modelcontextprotocol/go-sdk` | Go | All versions | 1.3.1 |
| `github.com/modelcontextprotocol/go-sdk` | Go | All versions | 1.3.1 |
| `github.com/modelcontextprotocol/go-sdk` | Go | All versions | 1.3.1 |

Products: Lfprojects Mcp Go Sdk.

## How do I fix CVE-2026-27896?

Upgrade github.com/modelcontextprotocol/go-sdk to 1.3.1 or later.

## Is CVE-2026-27896 being exploited?

There is no known exploitation of CVE-2026-27896 in the wild and no public exploit code.

## Other vulnerabilities in modelcontextprotocol go-sdk

- [CVE-2026-34742](https://insights.sbomify.com/CVE-2026-34742/): Model Context Protocol Go SDK: DNS Rebinding Protection Disabled by Default for Servers Running on Localhost (high 7.6)
- [CVE-2026-33252](https://insights.sbomify.com/CVE-2026-33252/): MCP Go SDK Allows Cross-Site Tool Execution for HTTP Servers without Authorizatrion (high 7.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-27896 and fix it if it is affected.

What it is: CVE-2026-27896 is a high-severity vulnerability in modelcontextprotocol go-sdk, published on 26 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-27896 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/modelcontextprotocol/go-sdk (Go): affected all versions; fixed in 1.3.1
- github.com/modelcontextprotocol/go-sdk (Go): affected all versions; fixed in 1.3.1
- github.com/modelcontextprotocol/go-sdk (Go): affected all versions; fixed in 1.3.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-27896/ (Markdown: https://insights.sbomify.com/CVE-2026-27896/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27896
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27896
- OSV: https://osv.dev/vulnerability/CVE-2026-27896

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-27896/
Summaries are generated and may contain mistakes; the official description above is the record.