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

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), Lfprojects Mcp Typescript Sdk, @modelcontextprotocol/sdk (npm). Vendors: Anthropic, Lfprojects, 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
    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

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