modelcontextprotocol go-sdk up to 1.3.0 MCP Protocol case sensitivity

Summaryinfo

A vulnerability identified as critical has been detected in modelcontextprotocol go-sdk up to 1.3.0. The impacted element is an unknown function of the component MCP Protocol. Performing a manipulation results in case sensitivity. This vulnerability is reported as CVE-2026-27896. The attack is possible to be carried out remotely. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in modelcontextprotocol go-sdk up to 1.3.0. It has been classified as critical. This affects some unknown functionality of the component MCP Protocol. The manipulation with an unknown input leads to a case sensitivity vulnerability. CWE is classifying the issue as CWE-178. The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-27896 since 02/24/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Neither technical details nor an exploit are publicly available.

Upgrading to version 1.3.1 eliminates this vulnerability. Applying the patch 7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• modelcontextprotocol

Name

• go-sdk

Version

• 1.0
• 1.1
• 1.2
• 1.3.0

License

• open-source

Website

• Product: https://github.com/modelcontextprotocol/go-sdk/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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