# CVE-2026-27896: Model Context Protocol Go SDK improper case sensitivity in JSON-RPC parsing

Source: https://junglewise.ai/threats/cve-2026-27896-model-context-protocol-go-sdk-improper-case-sensitivity-in-json  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-02-26. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-27896 · Severity: high · CVSS 7.5 · Published 2026-02-26

The Model Context Protocol (MCP) Go SDK, used for building integrations between AI models and tools, contained a flaw in how it processed incoming messages. Because it used a standard Go library that ignores letter casing in data fields, it would accept messages that technically violate the protocol's rules. This could allow a malicious actor to bypass security filters or inspection tools that expect strictly formatted data, potentially leading to unauthorized actions or inconsistent security enforcement across different systems.

Technologies: [github.com/modelcontextprotocol/go-sdk (Go)](https://junglewise.ai/threats/technologies/github-com-modelcontextprotocol-go-sdk). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Lfprojects](https://junglewise.ai/threats/vendors/lfprojects).

## Quick answers

**Is CVE-2026-27896 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-27896 being exploited in the wild.

**How severe is CVE-2026-27896?** High severity, with a CVSS score of 7.5, published 26 February 2026.

**Which products and versions are affected?** github.com/modelcontextprotocol/go-sdk (Go) SEMVER:introduced=0,fixed=1.3.1.

**Is there a fix for CVE-2026-27896?** Yes. github.com/modelcontextprotocol/go-sdk (Go): upgrade to 1.3.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-27896 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-27896?** Check whether you run github.com/modelcontextprotocol/go-sdk (Go) or Lfprojects Mcp Go Sdk at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/modelcontextprotocol/go-sdk (Go): upgrade to 1.3.1 or later

  ```sh
  go get github.com/modelcontextprotocol/go-sdk@v1.3.1
  ```


## Technical details

The Go MCP SDK utilized the standard 'encoding/json.Unmarshal' library, which performs case-insensitive matching of JSON keys to struct field tags. This behavior violates the JSON-RPC 2.0 specification requiring exact field names and includes unexpected Unicode folding (e.g., matching 'paramſ' to 'params'). A remote attacker can send protocol messages with non-standard field casing to bypass intermediary inspection layers (like proxies or WAFs) that enforce exact-match policies. This also creates cross-implementation inconsistencies with TypeScript and Python SDKs which are case-sensitive. The issue was resolved in version 1.3.1 by migrating to a case-sensitive decoder.

## Affected products

- modelcontextprotocol go-sdk < 1.3.1

## Identifiers

- CVE-2026-27896
- GHSA-wvj2-96wp-fq3f
- GO-2026-4569

## Timeline

- 2026-02-18: patched: Case-sensitive decoder introduced in commit 7b8d81c
- 2026-02-25: advisory: GHSA-wvj2-96wp-fq3f published
- 2026-02-26: disclosed: CVE-2026-27896 published

## References

- [https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0](https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0)
- [https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f](https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96wp-fq3f)
- [https://access.redhat.com/security/cve/CVE-2026-27896](https://access.redhat.com/security/cve/CVE-2026-27896)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2442903](https://bugzilla.redhat.com/show_bug.cgi?id=2442903)
- [https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json)

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- [GO-2026-4770 - Improper handling of null Unicode character when parsing JSON in github.com/modelcontextprotocol/go-sdk](https://junglewise.ai/threats/go-2026-4770-improper-handling-of-null-unicode-character-when-parsing-6cff319e)
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- [Improper handling of null Unicode character when parsing JSON in github.com/modelcontextprotocol/go-sdk](https://junglewise.ai/threats/improper-handling-of-null-unicode-character-when-parsing-json-in-github-27191820)