AI agent vulnerability · MCP TypeScript SDK

CVE-2026-0621: Anthropic's MCP TypeScript SDK has a ReDoS vulnerability

highCVSS 4.0 8.7GHSA-8r9q-7v3j-jr4g

Impact A ReDoS vulnerability in the UriTemplate class allows attackers to cause denial of service. The partToRegExp() function generates a regex pattern with nested quantifiers (([^/]+(?:,[^/]+))) for exploded template variables (e.g., {/id}, {?tags}), causing catastrophic backtracking on malicious input.

Published Jan 5, 2026 · Updated Jul 15, 2026

Affected packages

@modelcontextprotocol/sdk

Ecosystem: npm
Affected: >= 1.3.0, < 1.25.2
Fixed in: 1.25.2
Check: npm ls @modelcontextprotocol/sdk

Advisory details

From the GitHub Advisory Database (CC BY 4.0).

Impact

A ReDoS vulnerability in the UriTemplate class allows attackers to cause denial of service. The partToRegExp() function generates a regex pattern with nested quantifiers (([^/]+(?:,[^/]+)*)) for exploded template variables (e.g., {/id*}, {?tags*}), causing catastrophic backtracking on malicious input.

Who is affected: MCP servers that register resource templates with exploded array patterns and accept requests from untrusted clients.

Attack result: An attacker sends a crafted URI via resources/read request, causing 100% CPU utilization, server hang/crash, and denial of service for all clients.

Affected Versions

All versions of @modelcontextprotocol/sdk prior to the patched release.

Patches

v1.25.2 contains b392f02ffcf37c088dbd114fedf25026ec3913d3 the fix modifies the regex pattern to prevent backtracking.

Workarounds

Avoid using exploded patterns ({/id*}, {?tags*}) in resource templates
Implement request timeouts and rate limiting
Validate URIs before processing to reject suspicious patterns

Credited to Clashsoft, albertabiev1.

References

How this page is made: the data panel comes from the GitHub Advisory Database, CISA’s KEV catalog, the NVD and FIRST EPSS, and updates twice a day. No analysis has been written for this entry yet. Methodology

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