CVE-2026-0621 identifies a regular expression denial of service (ReDoS) vulnerability in the MCP TypeScript SDK developed by Anthropic, specifically in versions up to and including 1.25.1. The vulnerability exists in the UriTemplate class, which processes URI templates compliant with RFC 6570. When handling exploded array patterns, the SDK dynamically generates a regular expression that contains nested quantifiers. These nested quantifiers can cause catastrophic backtracking when processing specially crafted input URIs, leading to excessive CPU consumption and effectively causing the Node.js process to become unresponsive. This is a classic ReDoS scenario where the complexity of the regex grows exponentially with malicious input, resulting in denial of service. The vulnerability requires no authentication or user interaction, making it remotely exploitable over the network. While no known exploits have been reported in the wild, the high CVSS score of 8.7 reflects the significant impact on availability and ease of exploitation. The vulnerability affects any application or service that incorporates the MCP TypeScript SDK for URI processing, particularly those exposing URI template parsing functionality to untrusted input. Since the SDK is used in TypeScript/Node.js environments, web services, APIs, or microservices leveraging this SDK are vulnerable. The lack of a patch link indicates that a fix may not yet be publicly available, necessitating interim mitigations.

The primary impact of CVE-2026-0621 is denial of service through resource exhaustion, specifically CPU consumption leading to unresponsiveness of Node.js processes. For European organizations, this can disrupt critical web services, APIs, or microservices that rely on the MCP TypeScript SDK for URI processing. This may result in service outages, degraded user experience, and potential financial losses. Organizations in sectors such as finance, e-commerce, telecommunications, and government services that depend on Node.js-based infrastructure are particularly at risk. The vulnerability could be exploited by attackers to launch targeted DoS attacks, potentially as part of larger campaigns to disrupt operations or as a vector for distraction during other attacks. Since the vulnerability does not compromise confidentiality or integrity directly, the impact is primarily on availability. However, prolonged outages can indirectly affect trust and compliance with service-level agreements and regulatory requirements. The absence of known exploits in the wild suggests limited immediate risk, but the ease of exploitation and high severity score warrant proactive mitigation.

1. Monitor Anthropic's official channels for patches addressing CVE-2026-0621 and apply updates promptly once available. 2. In the interim, implement strict input validation to sanitize and reject suspicious or overly complex URI patterns before they reach the vulnerable UriTemplate processing code. 3. Apply rate limiting and throttling on endpoints that process URI templates to reduce the risk of repeated exploitation attempts causing resource exhaustion. 4. Consider isolating services using the MCP TypeScript SDK in separate containers or processes with resource limits to contain potential DoS impact. 5. Employ runtime monitoring and alerting for abnormal CPU usage patterns in Node.js processes handling URI templates. 6. Review and audit usage of the MCP TypeScript SDK in your environment to identify exposed services and assess risk exposure. 7. If feasible, temporarily replace or wrap the vulnerable URI processing functionality with safer alternatives or custom implementations that avoid vulnerable regex patterns. 8. Educate development teams about ReDoS risks and secure coding practices related to regular expressions.