CVE-2026-0621 identifies a Regular Expression Denial of Service (ReDoS) vulnerability in the MCP TypeScript SDK developed by Anthropic, specifically affecting versions up to and including 1.25.1. The vulnerability resides in the UriTemplate class, which processes URI templates compliant with RFC 6570, particularly exploded array patterns. The SDK dynamically generates regular expressions to match URIs, but these regex patterns contain nested quantifiers that can cause catastrophic backtracking when processing specially crafted malicious inputs. This backtracking leads to excessive CPU usage, causing the Node.js process running the SDK to become unresponsive and effectively denying service. The vulnerability can be exploited remotely without any authentication or user interaction, as it only requires supplying a malicious URI to the vulnerable component. The CVSS 4.0 base score of 8.7 (high severity) reflects the ease of exploitation and the significant impact on availability. No patches or fixes are currently linked, and no known exploits have been reported in the wild yet. The vulnerability is classified under CWE-1333 (Inefficient Regular Expression Complexity), highlighting the root cause as inefficient regex design. This issue is particularly critical for applications that rely on the MCP TypeScript SDK for URI processing in Node.js environments, as it can be leveraged to disrupt service availability.

The primary impact of CVE-2026-0621 is denial of service through resource exhaustion, specifically CPU consumption due to catastrophic backtracking in regex processing. Organizations using the vulnerable MCP TypeScript SDK in their Node.js applications may experience unresponsive services or crashes when processing maliciously crafted URIs. This can lead to downtime, degraded user experience, and potential loss of business continuity. Since the vulnerability requires no authentication or user interaction, it can be exploited by unauthenticated remote attackers, increasing the attack surface. Services exposed to the internet that parse URIs using this SDK are at higher risk. The disruption could affect web applications, APIs, and microservices relying on the SDK, potentially impacting critical business operations. Additionally, the inability to process legitimate requests during an attack could lead to cascading failures in dependent systems. Although no known exploits are reported yet, the high severity and ease of exploitation make it a significant threat to organizations worldwide.

To mitigate CVE-2026-0621, organizations should prioritize updating the MCP TypeScript SDK to a patched version once available from Anthropic. In the absence of an official patch, developers should avoid processing untrusted or user-supplied URI templates that trigger the vulnerable regex logic, especially those involving exploded array patterns per RFC 6570. Implement input validation and sanitization to reject suspicious URI patterns before they reach the vulnerable code path. Consider applying regex timeouts or using safer regex libraries that limit backtracking. Monitoring application performance and setting CPU usage thresholds can help detect and mitigate ongoing attacks. Employing Web Application Firewalls (WAFs) with custom rules to block malicious URI patterns may provide temporary protection. Additionally, isolating the vulnerable component in a sandboxed environment can limit the impact of exploitation. Finally, maintain awareness of updates from the vendor and apply security advisories promptly.