How severe is CVE-2026-34742?

No CVSS score has been assigned to CVE-2026-34742 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-34742?

CVE-2026-34742 affects the following packages: github.com/modelcontextprotocol/go-sdk (Go). Ecosystems affected: Go.

How do I fix CVE-2026-34742?

Update github.com/modelcontextprotocol/go-sdk to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34742 is resolved across your whole dependency graph.

How do I detect CVE-2026-34742 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/modelcontextprotocol/go-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-34742 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-34742 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-34742 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-34742?

CVE-2026-34742 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 41th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-34742?

CVE-2026-34742 is classified as CWE-1188 (CWE-1188). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-34742 published, and has it been updated?

CVE-2026-34742 was published on April 2, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-34742 — go-sdk

Fix: modelcontextprotocol/go-sdk@67bd3f2

CVE-2026-34742 is a CWE-1188 vulnerability in github.com/modelcontextprotocol/go-sdk. A fix is available for github.com/modelcontextprotocol/go-sdk — see the affected versions and patch details below.

Model Context Protocol Go SDK: DNS Rebinding Protection Disabled by Default for Servers Running on Localhost

Also known asGHSA-xw59-hvm2-8pj6GO-2026-5771

Published

Apr 2, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34742.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs41th percentile — riskier than 41% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐹github.com/modelcontextprotocol/go-sdk

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

The Model Context Protocol (MCP) Go SDK does not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server is run on localhost without authentication with StreamableHTTPHandler or SSEHandler, a malicious website could exploit DNS rebinding to bypass same-origin policy restrictions and send requests to the local MCP server. This could allow an attacker to invoke tools or access resources exposed by the MCP server on behalf of the user in those limited circumstances.

Note that running HTTP-based MCP servers locally without authentication is not recommended per MCP security best practices. This issue does not affect servers using stdio transport.

Servers created via StreamableHTTPHandler or SSEHandler now have this protection enabled by default when binding to localhost. Users are advised to update to version 1.4.0 to receive this automatic protection.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/modelcontextprotocol/go-sdk`	all versions	1.4.0`go get github.com/modelcontextprotocol/go-sdk@v1.4.0`

Affected Products

1 product · 1 configurations

Application

mcp go sdklfprojects

< 1.4.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/modelcontextprotocol/go-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/modelcontextprotocol/go-sdk to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34742 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Source: Red Hat security advisory for CVE-2026-34742 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat OpenShift Dev Spaces 3.28	`devspaces/udi-rhel9:1779829736`	RHSA-2026:21772

Frequently Asked Questions

The Model Context Protocol (MCP) Go SDK does not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server is run on localhost without authentication with `StreamableHTTPHandler` or `SSEHandler`, a malicious website could exploit DNS rebinding to bypass same-origin policy restrictions and send requests to the local MCP server. This could allow an attacker to invoke tools or access resources exposed by the MCP server on behalf of the user in those limited circumstances. Note that running HTTP-based MCP servers locally without authentication is not recomme

O3 Security · Impact-Aware SCA

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