CVE-2026-53711: MCP Context Forge Admin UI XSS Vulnerability

CVE-2026-53711 Overview

CVE-2026-53711 is a stored Cross-Site Scripting (XSS) vulnerability in IBM's MCP Context Forge, which provides an MCP Admin UI for managing gateway resources. The Global Prompts feature stores prompt templates without sufficient output encoding before the MCP Admin UI renders them. An authenticated attacker can save markup containing script-capable event handlers, and the code executes when another user views the saved prompt. The flaw affects versions from 0.2.0 until 0.3.1 and is tracked under [CWE-79]. The issue is fixed in version 0.3.1.

Critical Impact
Stored script execution can alter the Admin UI, present phishing content, capture keystrokes available to the page, and act with the viewing user's browser session.

Affected Products

IBM MCP Context Forge 0.2.0 through versions prior to 0.3.1
MCP Admin UI (Global Prompts feature)
MCP Gateway deployments exposing the Admin UI to multiple authenticated users

Discovery Timeline

2026-10-13 - CVE-2026-53711 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-53711

Vulnerability Analysis

The Global Prompts feature in MCP Context Forge persists prompt templates supplied by authenticated users. When another user later loads these templates in the Admin UI, the server-rendered HTML includes the attacker-controlled content without adequate output encoding. Because the Admin UI accepts markup containing event handlers such as onerror or onload, the browser executes attacker-supplied JavaScript in the context of the viewing user. The stored nature of the flaw means a single malicious save persists across sessions and victims. Exploitation requires authenticated write access to prompts and user interaction by a subsequent viewer.

Root Cause

The root cause is missing HTML escaping on prompt template fields rendered by the MCP Admin UI. User-controlled strings reach the DOM as live markup instead of inert text, violating standard output-encoding practice for untrusted input [CWE-79]. The upstream fix in commit acd16196 introduces proper HTML escaping for admin UI user data rendering.

Attack Vector

An authenticated attacker with permission to create or edit Global Prompts stores a template containing script-capable markup. When any other authenticated user, including administrators, opens the prompt in the Admin UI, the embedded handler fires in their browser session. The attacker can then exfiltrate session data, perform actions as the victim via the Admin API, modify UI elements to stage phishing, or capture keystrokes within the page.

text

# Patch excerpt: Admin UI and Admin API disabled by default
-MCPGATEWAY_UI_ENABLED=true
+MCPGATEWAY_UI_ENABLED=false

-MCPGATEWAY_ADMIN_API_ENABLED=true
+MCPGATEWAY_ADMIN_API_ENABLED=false
# Source: https://github.com/IBM/mcp-context-forge/commit/acd16196158f9a393187551b21a2fc61650f2e3d

Detection Methods for CVE-2026-53711

Indicators of Compromise

Prompt template records in the gateway database containing HTML tags, <script> fragments, or inline event handlers such as onerror=, onload=, or onmouseover=.
Admin UI sessions issuing unexpected Admin API calls shortly after a user opens a Global Prompt view.
Outbound requests from browsers of Admin UI users to attacker-controlled domains referencing session tokens or form data.

Detection Strategies

Review the prompts table and related storage for Global Prompt entries that include markup rather than plain text.
Enable and inspect audit logs for prompt create and update operations, correlating authors with later viewer activity.
Deploy a Content Security Policy (CSP) in report-only mode to surface inline script execution attempts in the Admin UI.

Monitoring Recommendations

Alert on Admin API actions initiated immediately after Admin UI page loads of user-supplied prompts.
Monitor browser error telemetry and CSP violation reports for the Admin UI origin.
Track MCP Context Forge version strings across deployments to confirm all instances are running 0.3.1 or later.

How to Mitigate CVE-2026-53711

Immediate Actions Required

Upgrade MCP Context Forge to version 0.3.1 or later, available in the v0.3.1 release.
Audit all existing Global Prompts for markup or event handlers and remove or sanitize affected entries.
Rotate session tokens and credentials for any user who may have viewed a tampered prompt.
Restrict prompt authoring permissions to trusted users until the upgrade is verified in production.

Patch Information

The fix is delivered in MCP Context Forge 0.3.1 via commit acd16196, which adds proper HTML escaping for admin UI user data rendering. Additional context is available in the GitHub Security Advisory GHSA-x6gc-rm5j-55mw and the pull request discussion.

Workarounds

Disable the Admin UI by setting MCPGATEWAY_UI_ENABLED=false until the upgrade is applied.
Disable the Admin API endpoints by setting MCPGATEWAY_ADMIN_API_ENABLED=false where operationally feasible.
Place the Admin UI behind network restrictions so only trusted operators can reach it.

bash

# Disable Admin UI and Admin API in the gateway environment file
MCPGATEWAY_UI_ENABLED=false
MCPGATEWAY_ADMIN_API_ENABLED=false
# Source: https://github.com/IBM/mcp-context-forge/commit/acd16196158f9a393187551b21a2fc61650f2e3d