CVE-2026-278967.2HighCWE-178

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

modelcontextprotocol · go-sdk

Affected: modelcontextprotocol go-sdk · Exploitation: unlikely (under 1%)

Not in CyberSIXT coverage yet: no article names this CVE. This page shows the public record.

0.3%probability of exploitation in the next 30 days

22nd percentile

050100

Higher than 22% of scored CVEs

7.2HighCVSS 3.1

LowMediumHighCrit.

DescriptionCVE.org

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.

Affected products1

From the CVE record. A product or version that is not listed is unknown, not unaffected.

modelcontextprotocol · go-sdk< 1.3.1

On the Atlas

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Open the Atlas

Exploitation: Unlikely (under 1%). EPSS under 1%, or no score yet.
CISA KEV: Not listed
Public check: No public nuclei template
Weakness: Improper Handling of Case Sensitivity · Input Validation and Logic
Vendor: modelcontextprotocol
Published: February 2026
Band: Unlikely (under 1%), February 2026

CoverageNo articles yet

No CyberSIXT article names CVE-2026-27896 yet, so there is no coverage timeline, action brief or change log for it. Everything on this page comes from the public CVE record, CISA KEV, EPSS and the nuclei template index. Search the archive or see the CVEs in the news.