CVE-2026-81098 in telnyx-mcp
Summary
by MITRE • 08/27/2026
The Telnyx MCP server exposed its HTTP transport on every interface and did not require a caller credential. packages/mcp-server/src/http.ts served MCP on the root path with a listener bound to all interfaces and parsed the caller's authentication headers in a mode that did not fail when they were absent, so a request without any credential completed initialisation and dispatched tools. Dispatch forwarded the server's own stored credentials, the Telnyx API key and client secret together with the code-execution key, to the upstream endpoint, so an unauthenticated caller able to reach the port acted with them. The current code defaults the host to loopback, requires a server API key, and enforces it in middleware.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified involves a critical misconfiguration within the Telnyx MCP server that resulted in unauthorized access to sensitive operational capabilities. Specifically, the HTTP transport layer was bound to all network interfaces rather than being restricted to localhost or specific trusted networks. This architectural decision exposed the service endpoint on every available interface of the host machine, making it accessible from external networks if firewall rules permitted traffic to the port. Furthermore, the authentication mechanism implemented in the initial request handling logic failed to enforce mandatory credential validation. The server parsed caller authentication headers but operated in a permissive mode where missing or invalid credentials did not trigger an immediate rejection. Instead, the system proceeded with initialization and tool dispatching for requests that lacked any form of valid identity assertion.
This lack of access control allowed unauthenticated actors who could reach the exposed port to interact directly with the server's core functionality. The operational impact was severe because the middleware responsible for forwarding requests did not validate the caller's legitimacy before transmitting sensitive data upstream. Consequently, an attacker could dispatch tools and trigger actions using the server’s own stored credentials. These included the Telnyx API key, client secret, and code-execution keys, which were bundled with each request sent to the upstream endpoint. This effectively granted any external entity full administrative control over the associated services, enabling potential data exfiltration, service manipulation, or further lateral movement within the infrastructure by leveraging these high-privilege credentials.
From a classification perspective, this vulnerability aligns closely with CWE-287 Improper Authentication and CWE-434 Unrestricted Upload of File with Dangerous Type if file operations were involved, though primarily it represents a failure in access control logic often categorized under CWE-862 Missing Authorization. In terms of the MITRE ATT&CK framework, this behavior facilitates Initial Access through exposed services and Credential Access by allowing attackers to harvest or misuse stored API keys and secrets for subsequent attacks such as Service Exploitation (T1570) or Collection via compromised credentials. The root cause lies in a deviation from secure defaults where network binding should be restricted to loopback addresses unless explicitly required, and authentication must be enforced strictly at the entry point of any sensitive service interface.
To mitigate this vulnerability, immediate remediation steps involve updating the server configuration to bind the HTTP listener exclusively to 127.0.0.1 or localhost by default, thereby preventing external network access without explicit firewall rules. Additionally, the authentication middleware must be hardened to reject all requests that do not present valid and verified credentials before any initialization logic is executed. This ensures that no tools are dispatched and no upstream calls are made unless the caller’s identity has been positively authenticated. The current codebase already implements these safeguards by defaulting the host to loopback and requiring a server API key with strict enforcement in middleware, indicating that this was likely a configuration error or an outdated deployment state rather than a fundamental flaw in the application logic itself. Regular security audits of network binding configurations and authentication flows are recommended to prevent recurrence.