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Vulnerability Database/CVE-2026-11918

CVE-2026-11918: IBM ContextForge MCP Gateway Auth Bypass

CVE-2026-11918 is an authentication bypass vulnerability in IBM ContextForge MCP Gateway that lets authenticated users circumvent security controls through nested payload manipulation. This article covers technical details, affected versions, impact analysis, and mitigation strategies.

Published:

CVE-2026-11918 Overview

CVE-2026-11918 affects IBM ContextForge MCP Gateway versions <= 1.0.4. The flaw allows an authenticated user to bypass protection mechanisms because the gateway performs incomplete recursive inspection of nested payload content. The weakness is categorized under [CWE-184: Incomplete List of Disallowed Inputs].

An authenticated attacker can craft nested payloads that evade content inspection routines. Once inspection is bypassed, the attacker can influence confidentiality and integrity of data processed by the Model Context Protocol (MCP) gateway. Availability is not directly impacted.

Critical Impact

Authenticated users can smuggle disallowed content past ContextForge MCP Gateway inspection by nesting it inside payload structures the recursive scanner does not fully traverse.

Affected Products

  • IBM ContextForge MCP Gateway versions <= 1.0.4
  • IBM mcp-context-forge component
  • Deployments exposing MCP Gateway endpoints to authenticated users

Discovery Timeline

  • 2026-09-15 - CVE-2026-11918 published to the National Vulnerability Database (NVD)
  • 2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-11918

Vulnerability Analysis

IBM ContextForge MCP Gateway acts as a broker for Model Context Protocol traffic between clients and backend AI services. The gateway inspects inbound payloads to enforce content and policy restrictions. The inspection routine walks payload structures to identify disallowed inputs.

The recursive traversal is incomplete. Certain nested structures inside the payload are not fully descended, leaving embedded content unchecked. An authenticated user who understands the payload schema can position disallowed content in these unreachable nodes. The gateway forwards the payload as if it passed inspection.

The impact is limited to authenticated principals with low privileges. Exploitation does not require user interaction and can be performed over the network. Confidentiality and integrity are affected at a low level because the bypass permits content the policy engine was intended to block.

Root Cause

The defect stems from a denylist enforcement pattern classified as [CWE-184]. The recursive inspector maintains an incomplete traversal of nested payload nodes. Handlers for specific container types do not recurse into their children, creating blind spots that circumvent the policy check.

Attack Vector

An attacker authenticates to the MCP Gateway with valid credentials. The attacker submits an MCP request whose payload nests disallowed content inside a container type the inspector fails to descend into. The gateway approves the request and passes the payload to downstream MCP services. See the IBM Support Page for vendor-supplied technical detail.

No verified proof-of-concept code has been published. Refer to the vendor advisory for payload schema details and reproduction guidance.

Detection Methods for CVE-2026-11918

Indicators of Compromise

  • MCP Gateway requests containing deeply nested payload structures or unusual container types that policy inspection normally rejects
  • Downstream MCP service logs showing content patterns that should have been blocked at the gateway
  • Authenticated sessions issuing MCP requests with payload shapes that deviate from the application baseline

Detection Strategies

  • Compare gateway-accepted payloads against downstream-observed content to identify inspection gaps
  • Alert on MCP requests whose payload depth or nesting count exceeds normal application ranges
  • Correlate authenticated user activity with content categories the policy engine is configured to block

Monitoring Recommendations

  • Enable verbose logging on the ContextForge MCP Gateway inspection module and forward logs to a centralized analytics platform
  • Track per-account rates of MCP requests containing nested payload types, and baseline normal usage
  • Monitor for schema anomalies in MCP payloads such as unexpected container wrappers around string or tool-call fields

How to Mitigate CVE-2026-11918

Immediate Actions Required

  • Upgrade IBM ContextForge MCP Gateway to a version later than 1.0.4 as directed by the IBM Support Page
  • Audit accounts with authenticated access to the MCP Gateway and revoke credentials that are no longer required
  • Review recent MCP Gateway traffic for nested payloads that may have bypassed inspection

Patch Information

IBM has published remediation guidance on the IBM Support Page for mcp-context-forge. Apply the fixed release above version 1.0.4 on every gateway instance. Verify the deployed version after upgrade and restart dependent MCP services.

Workarounds

  • Restrict MCP Gateway authentication to a minimum set of trusted service accounts until the patched version is deployed
  • Enforce payload size and nesting depth limits at an upstream reverse proxy to reduce the exploitable surface
  • Apply allowlist-based content policies at the downstream MCP service layer as a defense-in-depth control against denylist bypasses
bash
# Configuration example
# Verify installed ContextForge MCP Gateway version
pip show mcp-context-forge | grep -i version

# Upgrade to a fixed release above 1.0.4
pip install --upgrade 'mcp-context-forge>1.0.4'

Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.

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