Bug 2442903 (CVE-2026-27896)

Summary: CVE-2026-27896 modelcontextprotocol/go-sdk: improper handling of case sensitivity
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: anpicker, bparees, dfreiber, dhanak, drosa, drow, dsimansk, hasun, jburrell, jfula, jkoehler, jowilson, kingland, kverlaen, lphiri, mnovotny, nyancey, ometelka, ptisnovs, sausingh, syedriko, vkumar, xdharmai
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Go MCP SDK. This issue occurs due to an improper handling of case sensitivity during JSON-RPC message parsing, specifically in the matching of JSON keys to struct field tags. This behavior violates the JSON-RPC 2.0 specification, which explicitly requires case-sensitive field name matching. A malicious MCP peer able to send protocol messages with non-standard field casing can potentially bypass intermediary inspection, allowing attackers to smuggle payloads past upstream filters and cause cross-implementation inconsistency.
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Description OSIDB Bzimport 2026-02-26 01:02:02 UTC 
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.