The vulnerability is a regular expression denial of service in the UriTemplate class of the Anthropic MCP TypeScript SDK. When processing RFC 6570 exploded array patterns, the SDK builds a dynamic regular expression that contains nested quantifiers. An attacker can craft a URI containing a large exploded array pattern that forces catastrophic backtracking, exhausting CPU resources and rendering the Node.js process unresponsive. The impact is a denial of service aimed at disrupting application availability.

Anthropic’s MCP TypeScript SDK versions up to and including 1.25.1 are affected. The SDK is typically used in Node.js environments to resolve or match URIs against templates. Any deployment that integrates these SDK versions, regardless of the specific application, is vulnerable if it processes user‑supplied URIs containing exploded array patterns.

The CVSS score is 8.7, indicating a high severity. The EPSS score is less than 1%, showing a low probability of exploitation at present, and the vulnerability is not listed in CISA’s KEV catalog. The attack requires only the ability to supply a crafted URI—no authentication or privileged access is necessary—making the exploit path straightforward for remote users who can invoke the affected SDK functionality. The primary vector is remote, via HTTP requests or any API surface that forwards URIs to the SDK for matching.