Overview
The genieacs-mcp package, specifically version 0.3.1, exposes a local Streamable HTTP MCP endpoint that accepts attacker-controlled Host and Origin headers. This vulnerability allows a malicious web page to use DNS rebinding to route browser requests to a victim's loopback MCP listener, effectively bypassing the browser-origin boundary.
Understanding the Vulnerability / Threat
Root Cause Analysis
The root cause of this vulnerability is the lack of validation for Host and Origin headers in the genieacs-mcp package. The package treats loopback HTTP as a safety boundary but does not enforce this boundary properly, allowing an attacker to exploit this vulnerability.
Attack Surface & Vector
The attack surface for this vulnerability is the default loopback HTTP mode of the genieacs-mcp package. An attacker can exploit this vulnerability by crafting a malicious web page that uses DNS rebinding to interact with the victim's local MCP server.
Exploitation Mechanics — Scenario Walkthrough
Scenario: Compromising a Local GenieACS MCP Server
1. Initial Position: An attacker crafts a malicious web page that will be used to exploit the vulnerability.
2. Triggering the Flaw: The attacker uses DNS rebinding to route browser requests to the victim's loopback MCP listener. The victim's browser, while visiting the malicious web page, makes requests to the MCP server with attacker-controlled Host and Origin headers.
3. What Breaks: The MCP server, not validating the Host and Origin headers, accepts the requests and initializes an MCP session. This allows the attacker to list available tools and invoke GenieACS operations through the server's configured ACS_URL.
4. Attacker's Prize: The attacker gains unauthorized access to the GenieACS system, allowing them to perform various actions such as device reboot, firmware download, TR-069 parameter changes, and more.
Real-World Impact
In a real-world deployment, this vulnerability can expose or modify CPE management state through GenieACS. An attacker can use this vulnerability to gain control of the GenieACS system, potentially leading to unauthorized access, data theft, or other malicious activities.
Detection & Defense
Immediate Mitigations
To mitigate this vulnerability, users should update to a version of genieacs-mcp that validates Host and Origin headers. Additionally, requiring a bearer token for HTTP transport, even on loopback, can help prevent exploitation.
Detection Strategies
Defenders can detect exploitation attempts by monitoring for suspicious requests to the MCP server, such as requests with unusual Host or Origin headers. Implementing security measures such as validating Host and Origin headers, requiring authentication for loopback listeners, and monitoring for suspicious activity can help prevent exploitation.
Long-Term Hardening
To prevent similar vulnerabilities in the future, it is essential to implement robust security measures, such as:
- Validating Host and Origin headers for all requests
- Requiring authentication for all listeners, including loopback
- Implementing proper access controls and monitoring for suspicious activity
Key Takeaways
- The genieacs-mcp package has a DNS rebinding vulnerability that allows a malicious web page to interact with a victim's local GenieACS MCP server.
- The vulnerability is caused by the lack of validation for Host and Origin headers.
- Exploitation can lead to unauthorized access and control of the GenieACS system.
- Mitigations include updating to a patched version, requiring authentication for loopback listeners, and monitoring for suspicious activity.
Sources
- GitHub Security Advisories: GHSA-cmwv-wf9p-p8wx
- CVE: CVE-2026-55637