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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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.
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 | 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 include 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
Sources
Also known as CLEANSTART-2026-DZ42058, CLEANSTART-2026-EM35887, GHSA-wvj2-96wp-fq3f, GO-2026-4569, MINI-ccx8-75xj-wcv6, MINI-r9gj-5wqm-qpq4.