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Back to active CVEs
High · CVSS 7.5CVE-2026-27896GHSA-WVJ2-96WP-FQ3F

MCP Go SDK Vulnerable to Improper Handling of Case SensitivityCVE-2026-27896

Answer in brief

CVE-2026-27896 records a High severity (CVSS 7.5) vulnerability in MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity. The current sources do not mark it as known exploited. The current feed maps modelcontextprotocol/go-sdk (generic), github.com/modelcontextprotocol/go-sdk (go). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Feb 26, 2026Updated Sep 16, 2026Source checked Nov 3, 2026First seen by HOL Jul 15, 2026Material review Sep 16, 2026
Upstream Advisory

Record context

Vulnerability class
Tool Poisoning
EPSS
Not reported
CWE IDs
CWE-178, CWE-436
Source
CVE List V5
Source checked
Nov 3, 2026
References
6 linked sources
Open source record

Key facts

Risk
High · CVSS 7.5
Exploitation
Not marked as known exploited
Affected software
2 mapped packages or products
Fix availability
Available

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps modelcontextprotocol/go-sdk (generic), github.com/modelcontextprotocol/go-sdk (go). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
modelcontextprotocol/go-sdkgeneric< 1.3.1Not reported
github.com/modelcontextprotocol/go-sdkgo>=0 <1.3.11.3.1

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for modelcontextprotocol/go-sdk, github.com/modelcontextprotocol/go-sdk.
  2. 2Review the reported fix. Update github.com/modelcontextprotocol/go-sdk to 1.3.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Feb 26, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Sep 16, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Jul 15, 2026

Sources and claim methodology

  • Source referenceaccess.redhat.com
  • Source referencebugzilla.redhat.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • Source referencesecurity.access.redhat.com
  • NVD vulnerability recordnvd.nist.gov
Upstream source 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.

Quoted source text, attributed separately from HOL analysis.