# CVE-2026-0621: Anthropic MCP TypeScript SDK ReDoS in UriTemplate

Source: https://junglewise.ai/threats/cve-2026-0621-anthropic-mcp-typescript-sdk-redos-in-uritemplate  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-01-05. Updated: 2026-07-15  
License: CC BY 4.0 with a link back

CVE-2026-0621 · Severity: high · CVSS 7.5 · Published 2026-01-05

The Anthropic Model Context Protocol (MCP) TypeScript SDK is a library used to build servers that connect AI models to local data and tools. A vulnerability in how the library handles certain web address templates allows an attacker to crash the server or make it unresponsive by sending a specially crafted request. This results in a total service outage for all users connected to the affected AI server.

Technologies: [Anthropic Model Context Protocol SDK (TypeScript)](https://junglewise.ai/threats/technologies/model-context-protocol-sdk-typescript), [Lfprojects Mcp Typescript Sdk](https://junglewise.ai/threats/technologies/mcp-typescript-sdk), [@modelcontextprotocol/sdk (npm)](https://junglewise.ai/threats/technologies/modelcontextprotocol-sdk). Vendors: [Anthropic](https://junglewise.ai/threats/vendors/anthropic), [Lfprojects](https://junglewise.ai/threats/vendors/lfprojects), [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-0621 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-0621 being exploited in the wild.

**How severe is CVE-2026-0621?** High severity, with a CVSS score of 7.5, published 5 January 2026.

**Which products and versions are affected?** Anthropic Model Context Protocol SDK (TypeScript) >= 1.3.0, < 1.25.2; @modelcontextprotocol/sdk (npm) >= 1.3.0, < 1.25.2, SEMVER:introduced=1.3.0,fixed=1.25.2.

**Is there a fix for CVE-2026-0621?** Yes. @modelcontextprotocol/sdk (npm): upgrade to 1.25.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-0621 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-0621?** Check whether you run Anthropic Model Context Protocol SDK (TypeScript), Lfprojects Mcp Typescript Sdk or @modelcontextprotocol/sdk (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- @modelcontextprotocol/sdk (npm): upgrade to 1.25.2 or later

  ```sh
  npm install @modelcontextprotocol/sdk@1.25.2
  ```


## Technical details

A Regular Expression Denial of Service (ReDoS) vulnerability exists in the UriTemplate class of the @modelcontextprotocol/sdk package. The root cause is located in the partToRegExp() function, which generates regex patterns with nested quantifiers when processing 'exploded' template variables (e.g., {/id*} or {?tags*}). An attacker can exploit this by sending a crafted URI via a 'resources/read' request, triggering catastrophic backtracking. This results in 100% CPU utilization and a complete hang or crash of the MCP server. The issue is fixed in version 1.25.2 by modifying the regex pattern to prevent backtracking.

## Affected products

- Anthropic @modelcontextprotocol/sdk >= 1.3.0, < 1.25.2

## Identifiers

- CVE-2026-0621
- GHSA-8r9q-7v3j-jr4g

## Timeline

- 2026-01-05: disclosed
- 2026-01-05: advisory
- 2026-01-05: patched: Fixed in version 1.25.2

## References

- [https://api.github.com/users/Clashsoft](https://api.github.com/users/Clashsoft)
- [https://github.com/Clashsoft](https://github.com/Clashsoft)
- [https://api.github.com/users/Clashsoft/gists%7B/gist_id%7D](https://api.github.com/users/Clashsoft/gists%7B/gist_id%7D)
- [https://api.github.com/users/Clashsoft/repos](https://api.github.com/users/Clashsoft/repos)
- [https://avatars.githubusercontent.com/u/4145923?v=4](https://avatars.githubusercontent.com/u/4145923?v=4)
- [https://api.github.com/users/Clashsoft/events%7B/privacy%7D](https://api.github.com/users/Clashsoft/events%7B/privacy%7D)

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- [MCP TypeScript client SDK credential leak via cross-origin redirects](https://junglewise.ai/threats/cve-2026-111769-mcp-typescript-client-sdk-http-client-transports-and-oauth)
- [MCP TypeScript SDK memory exhaustion via unbounded validation](https://junglewise.ai/threats/cve-2026-111768-mcp-typescript-sdk-one-http-request-within-the-default-size)
- [MCP TypeScript SDK bearer authentication token validation bypass](https://junglewise.ai/threats/cve-2026-111771-mcp-typescript-sdk-server-bearer-authentication-accepted-tokens)
- [MCP TypeScript SDK denial of service via unbounded request body and JSON-RPC batch](https://junglewise.ai/threats/mcp-typescript-sdk-http-server-transports-and-handlers-did-not-limit-c51dda65)
- [MCP TypeScript SDK missing authorization in experimental tasks](https://junglewise.ai/threats/cve-2026-111767-mcp-typescript-sdk-experimental-tasks-were-not-tied-to-the)