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Fencer Vulns
high 7.2

CVE-2026-27896 json

MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

published 2026-02-26 8 mo ago modified 2026-09-16 54 d ago also EUVD-2026-8792 , GHSA-wvj2-96wp-fq3f , GO-2026-4569

cvss
7.2
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 21%
public exploits
0
none known

Summary

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.

Patch / fix

  • go github.com/modelcontextprotocol/go-sdk 1.3.1
  • lfprojects mcp_go_sdk 1.3.1
  • govulncheck-vulndb openSUSE Leap 15.6 0.0.20260317T205859-150000.1.152.1

suse advisories SUSE-SU-2026:1042-1

  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-27896 updated cvss 7.2

modelcontextprotocol/go-sdk: improper handling of case sensitivity

  • Red Hat Migration Toolkit for Virtualization mtv-candidate/mtv-api-rhel9, mtv-candidate/mtv-operator-bundle, mtv-candidate/mtv-controller-rhel9, mtv-candidate/mtv-cli-download-rhel9, migration-toolkit-virtualization/mtv-cli-download-rhel9
    affected: workaround, no fix available, no fix planned “To mitigate this flaw, strictly enforce JSON-RPC case sensitivity before payload processing or harden upstream WAF and proxy rules to explicitly block improperly cased requests at the network edge.”
  • Red Hat OpenShift Lightspeed openshift-lightspeed/openshift-mcp-server-rhel9 Red Hat OpenShift Serverless openshift-serverless-1/kn-client-kn-rhel9, openshift-serverless-1/kn-plugin-func-func-util-rhel9
    affected: workaround, no fix available “To mitigate this flaw, strictly enforce JSON-RPC case sensitivity before payload processing or harden upstream WAF and proxy rules to explicitly block improperly cased requests at the network edge.”
  • Red Hat OpenShift AI (RHOAI) rhoai/odh-dashboard-rhel9, rhoai/odh-mod-arch-gen-ai-rhel9, rhoai/odh-mod-arch-model-registry-rhel9
    affected: workaround, no fix planned “To mitigate this flaw, strictly enforce JSON-RPC case sensitivity before payload processing or harden upstream WAF and proxy rules to explicitly block improperly cased requests at the network edge.”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
8 mo ago

Exploit prediction

0.29% chance of exploitation in the next 30 days, higher than 21% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 7.0 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
CISA ADP 3.1 7.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
CNA 4.0 7.0 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
CNA 3.1 7.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
go github.com/modelcontextprotocol/go-sdk
<1.3.1
1.3.1

Affected products

  • modelcontextprotocol go-sdk
  • Red Hat Migration Toolkit for Virtualization
  • Red Hat OpenShift Lightspeed
  • Red Hat OpenShift Serverless
  • Red Hat Red Hat OpenShift AI (RHOAI)

Classification

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

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.