CVE-2026-0621: Anthropic's MCP TypeScript SDK has a ReDoS vulnerability
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.
What it is
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.
How to check
- Affected: @modelcontextprotocol/sdk (npm) >= 1.3.0, < 1.25.2.
- Compare your installed version with the range above (command below).
How to fix
- Upgrade @modelcontextprotocol/sdk to 1.25.2 or later.
npm ls @modelcontextprotocol/sdkSources
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