CVE-2026-51994 in mcp-remote
Summary
by MITRE • 09/24/2026
mcp-remote versions 0.1.32 through 0.1.38 are vulnerable to Server-Side Request Forgery (SSRF) via the resource_metadata URL extracted from a remote MCP server's WWW-Authenticate header
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Analysis
by VulDB Data Team • 09/24/2026
The vulnerability identified in mcp-remote versions ranging from 0.1.32 to 0.1.38 represents a critical security flaw classified as Server-Side Request Forgery, commonly referenced under the CWE identifier CWE-918. This specific implementation weakness arises from the improper validation and sanitization of user-controllable input derived from HTTP response headers during the authentication handshake process with remote Model Context Protocol servers. The core technical failure occurs when the application extracts a URL value from the WWW-Authenticate header provided by an external server and subsequently uses this extracted resource_metadata URL to initiate new network requests without adequate verification of the target destination. This lack of rigorous input validation allows an attacker who controls or can manipulate the remote MCP server's response headers to inject arbitrary URLs that direct the vulnerable application to make connections on behalf of itself, effectively bypassing local security boundaries and internal network restrictions.
From a technical perspective, the flaw exploits the trust relationship established during the initial authentication phase. When mcp-remote communicates with a remote server, it processes various header fields to establish context and authenticate subsequent requests. The specific code path responsible for parsing the WWW-Authenticate header fails to enforce strict allow-listing of domains or protocols for the resource_metadata URL field. Consequently, if an attacker configures their malicious MCP server to return a specially crafted WWW-Authenticate header containing a URL pointing to internal services such as cloud metadata endpoints like 169.254.169.254, local database ports, or other sensitive intranet resources, the vulnerable client will blindly follow this directive. This behavior enables the attacker to perform port scanning of internal networks, exfiltrate sensitive data from accessible services, and potentially pivot further into protected network segments by leveraging the application's outbound connection capabilities as a proxy mechanism.
The operational impact of this SSRF vulnerability is severe, particularly in environments where mcp-remote is deployed within corporate intranets or cloud infrastructure that relies on instance metadata for authentication and configuration retrieval. An attacker can leverage this flaw to access sensitive information stored in local service endpoints, such as AWS EC2 instance metadata which often contains temporary security credentials, IAM role ARNs, and other confidential configuration details. Beyond data exfiltration, the vulnerability facilitates reconnaissance activities by allowing the enumeration of internal services that are not exposed to the public internet but remain reachable from the host running mcp-remote. This can lead to a complete compromise of the underlying system if combined with other vulnerabilities or misconfigurations in adjacent services, effectively turning the client application into an attack vector against its own network environment.
To mitigate this risk, immediate action is required by upgrading the mcp-remote package to version 0.1.39 or later where these validation checks have been implemented and strengthened. In scenarios where upgrading is not immediately feasible, organizations should implement strict egress filtering rules at the firewall level to restrict outbound connections from hosts running vulnerable versions of the software to only known and necessary external endpoints. Additionally, deploying a web application firewall with SSRF detection capabilities can help identify and block requests targeting internal IP ranges or cloud metadata service addresses. It is also recommended that developers review any custom integrations involving HTTP header parsing to ensure that all URLs extracted from untrusted sources undergo rigorous validation against an allow-list of permitted domains and protocols before being used in subsequent network operations, thereby adhering to the principle of least privilege and secure by design standards outlined in OWASP guidelines.