Packages / @modelcontextprotocol/sdk / Security

HighVulnerabilityGHSA-8r9q-7v3j-jr4gCVE-2026-0621Published 5 Jan 2026

Anthropic's MCP TypeScript SDK has a ReDoS vulnerability

Affected releases · @modelcontextprotocol/sdk

  • 1.x1.3.0 – 1.25.1Fixed in 1.25.2

On 1.x, the newest line:

npm install @modelcontextprotocol/sdk@1.25.2

  1. Not affected

    0.4.0 – 1.2.0

    13 releases

  2. Affected

    1.3.0 – 1.25.1

    57 releases

  3. Fixed

    1.25.2 – 1.32.1

    12 releases, latest

82 stable releases of @modelcontextprotocol/sdk, oldest to newest.

Is your project affected?

Paste your package.json, or type the range you declare for @modelcontextprotocol/sdk. It is read in your browser and sent nowhere.

What happens

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 b392f02 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