CVE-2026-27896Network attack vector

Policy bypass via case-insensitive JSON key matching

Published Feb 26, 2026 · Updated Feb 26, 2026

Improper case handling in Model Context Protocol Go SDK before 1.3.1 allows remote attackers to bypass exact-key inspection via malformed JSON. The JSON-RPC and MCP decoders pass untrusted messages to encoding/json.Unmarshal, which matches JSON keys to tagged Go struct fields without exact case and folds certain Unicode characters to ASCII equivalents. A malicious MCP peer must reach an SDK-based client or server; accepted alternate spellings can evade exact-name policy layers and change how downstream components interpret the message.

CVSS severity7.0
High
EPSS probability0.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper case handling in Model Context Protocol Go SDK before 1.3.1 allows remote attackers to bypass exact-key inspection via malformed JSON. The JSON-RPC and MCP decoders pass untrusted messages to encoding/json.Unmarshal, which matches JSON keys to tagged Go struct fields without exact case and folds certain Unicode characters to ASCII equivalents. A malicious MCP peer must reach an SDK-based client or server; accepted alternate spellings can evade exact-name policy layers and change how downstream components interpret the message.

The record

CVE
CVE-2026-27896
Published
Feb 26, 2026
Updated
Feb 26, 2026
Vendor
Model Context Protocol
Product
Go SDK
Classifications
CWE-436, CWE-178
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Feb 26, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.3.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · Go SDKversion=< 1.3.1

Published CVSS scores

7.0GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-436Interpretation Conflict
CWE-178Improper Handling of Case Sensitivity

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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