CVE-2026-39885 Overview
CVE-2026-39885 is a Server-Side Request Forgery (SSRF) vulnerability affecting FrontMCP, a TypeScript-first framework for the Model Context Protocol (MCP). The vulnerability exists in the mcp-from-openapi library, which uses @apidevtools/json-schema-ref-parser to dereference $ref pointers in OpenAPI specifications without configuring URL restrictions or custom resolvers. This allows attackers to craft malicious OpenAPI specifications that force the server to make requests to internal network addresses, cloud metadata endpoints, or read local files during the initialize() call.
Critical Impact
Attackers can exploit this vulnerability to access internal network resources, retrieve sensitive cloud metadata (such as AWS instance credentials), or read local files on the server, leading to significant information disclosure.
Affected Products
- FrontMCP versions prior to 2.3.0
- mcp-from-openapi library (bundled with FrontMCP)
Discovery Timeline
- 2026-04-08 - CVE-2026-39885 published to NVD
- 2026-04-09 - Last updated in NVD database
Technical Details for CVE-2026-39885
Vulnerability Analysis
This vulnerability stems from insufficient input validation when processing OpenAPI specifications containing $ref pointers. The mcp-from-openapi library relies on @apidevtools/json-schema-ref-parser to resolve JSON schema references, but fails to implement proper URL restrictions or custom resolvers that would prevent access to sensitive resources.
When a malicious OpenAPI specification is processed, the parser blindly follows $ref values to their target URLs during the initialize() function call. This behavior can be weaponized by an attacker who supplies an OpenAPI spec with carefully crafted references pointing to internal services, cloud provider metadata endpoints (such as http://169.254.169.254/ for AWS), or local file paths using the file:// protocol.
The vulnerability is classified under CWE-918 (Server-Side Request Forgery), as the application accepts user-controlled input that influences outbound requests without proper validation.
Root Cause
The root cause lies in the absence of URL validation and access controls within the JSON schema dereferencing logic. The @apidevtools/json-schema-ref-parser library provides options for configuring custom resolvers and URL restrictions, but the mcp-from-openapi implementation did not leverage these security controls. As a result, the parser treats all $ref targets as trusted, regardless of whether they point to internal resources, cloud metadata services, or local filesystem paths.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:
- Crafting a malicious OpenAPI specification with $ref pointers targeting sensitive internal resources
- Submitting the specification to an application using the vulnerable FrontMCP library
- The initialize() function processes the specification and follows the malicious references
- Internal resources, cloud metadata, or local files are fetched and potentially exposed to the attacker
A malicious OpenAPI specification might contain $ref values pointing to internal network addresses like http://internal-api.local/admin, cloud metadata endpoints such as http://169.254.169.254/latest/meta-data/iam/security-credentials/, or local files via file:///etc/passwd. When processed by the vulnerable library, these references are resolved, allowing the attacker to exfiltrate sensitive data. For complete technical details, see the GitHub Security Advisory GHSA-v6ph-xcq9-qxxj.
Detection Methods for CVE-2026-39885
Indicators of Compromise
- Unusual outbound HTTP requests from application servers to internal IP ranges (e.g., 10.x.x.x, 172.16.x.x, 192.168.x.x)
- Requests to cloud metadata endpoints (169.254.169.254)
- Attempts to access local files via file:// protocol from the application process
- OpenAPI specifications containing suspicious $ref values in request payloads
Detection Strategies
- Monitor network traffic for outbound connections to internal IP ranges or cloud metadata endpoints originating from application servers
- Implement web application firewall (WAF) rules to detect and block OpenAPI specifications containing potentially malicious $ref values
- Deploy endpoint detection and response (EDR) solutions to identify unusual file access patterns from Node.js/TypeScript processes
- Review application logs for errors related to failed internal resource access attempts
Monitoring Recommendations
- Configure network monitoring to alert on connections to the cloud metadata IP address 169.254.169.254 from application workloads
- Implement egress filtering to restrict outbound connections from application servers to only necessary external endpoints
- Set up file integrity monitoring on sensitive system files that may be targeted via local file read attacks
- Enable verbose logging for the mcp-from-openapi library to capture $ref resolution attempts
How to Mitigate CVE-2026-39885
Immediate Actions Required
- Upgrade FrontMCP to version 2.3.0 or later immediately
- Audit any OpenAPI specifications processed by the application for suspicious $ref values
- Implement network-level controls to block outbound requests to internal networks and cloud metadata endpoints from application servers
- Consider temporarily disabling features that process untrusted OpenAPI specifications until the upgrade is complete
Patch Information
The vulnerability has been fixed in FrontMCP version 2.3.0. The patch implements proper URL restrictions and custom resolvers for the JSON schema dereferencing process, preventing the parser from accessing internal resources, cloud metadata endpoints, or local files.
For detailed patch information, refer to:
Workarounds
- Implement an allowlist of trusted domains for $ref resolution at the application level before passing specifications to the library
- Deploy network segmentation to isolate application servers from sensitive internal resources
- Use a reverse proxy or API gateway to sanitize incoming OpenAPI specifications, stripping or validating $ref values before processing
- Block access to cloud metadata endpoints (169.254.169.254) at the network level for application workloads
# Example iptables rule to block cloud metadata endpoint access
iptables -A OUTPUT -d 169.254.169.254 -j DROP
# Example for blocking internal network ranges from application server
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
