CVE-2026-34742 — go-sdk
Fix: modelcontextprotocol/go-sdk@67bd3f2CVE-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
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
github.com/modelcontextprotocol/go-sdkReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/modelcontextprotocol/go-sdk | all versions | 1.4.0go get github.com/modelcontextprotocol/go-sdk@v1.4.0 |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34742 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34742. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat OpenShift Dev Spaces 3.28 | devspaces/udi-rhel9:1779829736 | RHSA-2026:21772 |
Frequently Asked Questions
Is CVE-2026-34742 in your dependencies?
O3 Security finds CVE-2026-34742 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.