CVE-2026-9739
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DNS Rebinding in Toolbox SSE Implementation

Publication date: 2026-05-27

Last updated on: 2026-05-27

Assigner: Google Inc.

Description
Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790). During the beta phase, we implemented `allowed-origins` and `allowed-hosts` flags to align with MCP security guidelines. However, the hardcoded `Access-Control-Allow-Origin: *` header in the SSE initialization handler was inadvertently retained. This vulnerability specifically impacts users connecting via Toolbox using SSE under specification v2024-11-05.
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Meta Information
Published
2026-05-27
Last Modified
2026-05-27
Generated
2026-06-17
AI Q&A
2026-05-28
EPSS Evaluated
2026-06-16
NVD
CVE-2026-9739
EUVD
EUVD-2026-32672
Affected Vendors & Products
Currently, no data is known.
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Exploitability
CWE
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KEV
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CWE ID Description
CWE-942 The product uses a web-client protection mechanism such as a Content Security Policy (CSP) or cross-domain policy file, but the policy includes untrusted domains with which the web client is allowed to communicate.
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Executive Summary

This vulnerability involves DNS rebinding attacks when using Server-Sent Events (SSE). During the beta phase, security measures such as `allowed-origins` and `allowed-hosts` flags were implemented to follow security guidelines. However, a hardcoded header `Access-Control-Allow-Origin: *` remained in the SSE initialization handler, which allows any origin to access the resource. This flaw specifically affects users connecting via Toolbox using SSE under specification v2024-11-05.

Impact Analysis

This vulnerability can allow attackers to perform DNS rebinding attacks, potentially bypassing same-origin policies. This means an attacker could trick a user's browser into interacting with internal or restricted network resources via the vulnerable SSE connection, leading to unauthorized access or data exposure.

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