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

> Attackers can cause denial of service in applications using the MCP TypeScript SDK by sending malicious URIs that trigger catastrophic backtracking in URI template processing.

- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 5 Jan 2026
- **Updated:** 8 Oct 2026
- **Also known as:** CLSA-2026-1789750026, ECHO-6585-27e0-f504, GHSA-8r9q-7v3j-jr4g, MINI-38mh-cmqx-4f24, MINI-434q-rwv3-qx4x, MINI-c8pg-4wwg-537f, MINI-f29q-8gc2-j42v, MINI-q38r-8wg3-7449, ROOT-APP-NPM-CVE-2026-0621

## What is CVE-2026-0621?

CVE-2026-0621 is a high-severity denial of service vulnerability in Anthropic MCP TypeScript SDK, published on 5 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2026-0621?

2 open source packages ship an affected version, including @modelcontextprotocol/sdk. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@modelcontextprotocol/sdk` | npm | From 1.3.0 | 1.25.2 |
| `@modelcontextprotocol/sdk` | npm | From 1.3.0 | 1.25.2 |

Products: Lfprojects Mcp Typescript Sdk.

## How do I fix CVE-2026-0621?

Upgrade @modelcontextprotocol/sdk to 1.25.2 or later.

## Is CVE-2026-0621 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-0621 exists.

## Other vulnerabilities in Anthropic MCP TypeScript SDK

- [CVE-2025-66414](https://insights.sbomify.com/CVE-2025-66414/): DNS Rebinding Protection Disabled by Default in Model Context Protocol TypeScript SDK for Servers Running on Localhost (high 7.6)
- [CVE-2026-25536](https://insights.sbomify.com/CVE-2026-25536/): @modelcontextprotocol/sdk has cross-client data leak via shared server/transport instance reuse (high 7.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-0621 and fix it if it is affected.

What it is: CVE-2026-0621 is a high-severity denial of service vulnerability in Anthropic MCP TypeScript SDK, published on 5 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-0621 exists.

Affected packages and fixed versions:
- @modelcontextprotocol/sdk (npm): affected from 1.3.0; fixed in 1.25.2
- @modelcontextprotocol/sdk (npm): affected from 1.3.0; fixed in 1.25.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-0621/ (Markdown: https://insights.sbomify.com/CVE-2026-0621/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-0621
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-0621
- OSV: https://osv.dev/vulnerability/CVE-2026-0621

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-0621/
Summaries are generated and may contain mistakes; the official description above is the record.