CVE-2026-0621 Overview
A Regular Expression Denial of Service (ReDoS) vulnerability has been identified in Anthropic's Model Context Protocol (MCP) TypeScript SDK versions up to and including 1.25.1. The vulnerability exists within the UriTemplate class when processing RFC 6570 exploded array patterns. The dynamically generated regular expression used during URI matching contains nested quantifiers that can trigger catastrophic backtracking on specially crafted inputs, resulting in excessive CPU consumption and potential service disruption.
Critical Impact
An attacker can exploit this vulnerability by supplying a malicious URI that causes the Node.js process to become unresponsive, leading to a denial of service condition affecting applications using the MCP TypeScript SDK.
Affected Products
- Anthropic MCP TypeScript SDK versions up to and including 1.25.1
- Applications utilizing the UriTemplate class for RFC 6570 URI template matching
- Node.js services implementing MCP protocol handling
Discovery Timeline
- 2026-01-05 - CVE CVE-2026-0621 published to NVD
- 2026-01-08 - Last updated in NVD database
Technical Details for CVE-2026-0621
Vulnerability Analysis
This vulnerability is classified under CWE-1333 (Inefficient Regular Expression Complexity). The root issue stems from how the UriTemplate class constructs regular expressions at runtime to match URI patterns defined by RFC 6570. When handling exploded array patterns (denoted by the * modifier in URI templates), the generated regex contains nested quantifiers that create an exponential number of potential matching paths.
Regular expression engines using backtracking algorithms are susceptible to this class of vulnerability. When a malicious input is crafted to maximize backtracking attempts, the regex engine can enter a state of catastrophic backtracking, consuming CPU resources for an extended period while attempting to find a match.
The attack is network-accessible and requires no authentication or user interaction, making it particularly concerning for public-facing services that process user-supplied URIs through the affected SDK component.
Root Cause
The vulnerability originates from the dynamic regex generation logic within the UriTemplate class. When processing RFC 6570 exploded array patterns, the code generates regular expressions with nested quantifiers (patterns like (a+)+ or (.*)*). These constructions create a combinatorial explosion of backtracking paths when the regex engine attempts to match certain input strings.
The lack of input validation and regex complexity analysis before pattern compilation allows attackers to trigger worst-case regex execution times through carefully constructed URI inputs.
Attack Vector
The attack vector is network-based, requiring an attacker to send a specially crafted URI to an application using the vulnerable SDK. The attack flow typically involves:
- Identifying a target application using the MCP TypeScript SDK for URI template processing
- Crafting a malicious URI string designed to trigger catastrophic backtracking in the regex engine
- Submitting the malicious URI through any endpoint that processes user-supplied URIs via the UriTemplate class
- The Node.js event loop becomes blocked during regex evaluation, causing the service to become unresponsive
The vulnerability does not require any special privileges and can be exploited by unauthenticated remote attackers. For detailed technical analysis, refer to the GitHub Issue Tracker Entry and the VulnCheck Security Advisory.
Detection Methods for CVE-2026-0621
Indicators of Compromise
- Abnormally high CPU utilization on Node.js processes running MCP SDK applications
- Unresponsive application endpoints that normally process URI template matching
- Application logs showing request timeouts or worker process restarts
- Monitoring alerts for event loop lag exceeding normal thresholds
Detection Strategies
- Implement application performance monitoring (APM) to detect CPU spikes during URI processing operations
- Monitor Node.js event loop latency metrics for anomalous blocking behavior
- Deploy web application firewall (WAF) rules to detect and block unusually long or malformed URI patterns
- Enable request timeout monitoring to identify requests that exceed expected processing times
Monitoring Recommendations
- Configure alerts for CPU utilization exceeding baseline thresholds on services using the MCP TypeScript SDK
- Implement request duration monitoring with alerting for requests exceeding normal processing times
- Monitor application restart frequencies which may indicate DoS conditions
- Review access logs for patterns of requests with abnormally long or repetitive URI segments
How to Mitigate CVE-2026-0621
Immediate Actions Required
- Upgrade the MCP TypeScript SDK to a version newer than 1.25.1 once a patched version is available
- Implement request timeouts at the application and infrastructure levels to limit DoS impact
- Deploy rate limiting on endpoints that process URI template matching to reduce attack surface
- Consider implementing input validation to reject abnormally long or malformed URI patterns before processing
Patch Information
Organizations using the MCP TypeScript SDK should monitor the GitHub Issue Tracker Entry for patch release announcements. Review the VulnCheck Security Advisory for additional mitigation guidance and updates on fix availability.
Workarounds
- Implement input length restrictions on URIs processed through the UriTemplate class
- Deploy regex execution timeouts at the application level if supported by the runtime environment
- Use reverse proxy or WAF rules to filter requests with potentially malicious URI patterns
- Consider isolating URI template processing to separate worker processes to prevent main application blocking
# Example: Configure request timeout in Node.js server
# Set server timeout to prevent long-running regex operations
# from blocking the application indefinitely
export NODE_OPTIONS="--max-old-space-size=512"
# Implement at application level with appropriate timeout handling
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
