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CVE Record

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

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.

HighCVSS 7.2Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Applications using the MCP Go SDK before 1.3.1 may accept protocol messages with incorrectly capitalized JSON-RPC field names. That can let a malicious MCP peer slip messages past tools that enforce the exact standard, creating inspection gaps and inconsistent behavior between components.

Executive priority

Prioritize remediation for internet-facing, multi-tenant, or partner-connected MCP deployments. The issue is not documented as actively exploited, but it affects trust boundaries and protocol inspection, making timely dependency updates appropriate.

Technical view

The SDK used Go encoding/json.Unmarshal for JSON-RPC and MCP parsing. Go matches JSON keys to struct tags case-insensitively, so fields like method could be accepted with alternate casing. JSON-RPC requires exact field names. Commit 7b8d81c replaced this with case-sensitive decoding; users should update to v1.3.1.

Likely exposure

Exposure is limited to systems that include modelcontextprotocol/go-sdk versions before 1.3.1, especially MCP clients, servers, or intermediaries that parse peer-supplied protocol messages. The bundle does not identify specific downstream products or Red Hat package impact.

Exploitation context

The source bundle does not show active exploitation, and KEV is false. The documented risk is a malicious MCP peer sending non-standard field casing that affected SDK versions silently accept, potentially bypassing intermediary inspection or causing cross-implementation inconsistency.

Researcher notes

Do not assume code execution or data theft from the cited sources. Focus validation on parser behavior, dependency reachability, and whether intermediary controls normalize or inspect JSON-RPC keys differently than the affected SDK.

Mitigation direction

  • Update modelcontextprotocol/go-sdk to v1.3.1 or later.
  • Regenerate SBOMs and dependency lock files after upgrading.
  • Check vendor advisories for downstream package status.
  • Ensure MCP intermediaries enforce exact JSON-RPC field names.

Validation and detection

  • Search dependency manifests for modelcontextprotocol/go-sdk versions before 1.3.1.
  • Confirm deployed artifacts include go-sdk v1.3.1 or later.
  • Review parser tests for exact JSON-RPC field-name handling.
  • Check vulnerability scanners for CVE-2026-27896 findings.
Prepared
Confidence
high
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-178: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-436: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-27896 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.2 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
6Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.2CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N3.92.7redhat-SADP
7CVSS 4.0HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N——GitHub_M

Vulnerability scoring details

Base CVSS 4.0 score

7High
CVSS 4.0 vector shape for CVE-2026-27896Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPmodelcontextprotocol/go-sdk: improper handling of case sensitivity
other:Red Hat severity ratingcvssV3_1
  • 2026-02-26T01:01:38.878Z: Reported to Red Hat.
  • 2026-02-26T00:47:46.967Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
modelcontextprotocolgo-sdk< 1.3.1Listed
Weakness

CWE details

CWE-178CWE-436

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-178 · source CWE mapping

Improper Handling of Case Sensitivity

Improper Handling of Case Sensitivity represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-436 · source CWE mapping

Interpretation Conflict

Interpretation Conflict represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs