CVE-2026-0621

Description

### 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 b392f02ffcf37c088dbd114fedf25026ec3913d3 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

How to fix CVE-2026-0621

To remediate CVE-2026-0621, upgrade the affected package to a fixed version below.

• —upgrade to 1.25.2 or later

Is CVE-2026-0621 being exploited?

Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.

Affected packages (1)

• from 0, < 1.25.2

CVSS scores

References (7)

• ADVISORYnvd.nist.gov/vuln/detail/CVE-2026-0621
• PATCHgithub.com/modelcontextprotocol/typescript-sdk
• WEBgithub.com/modelcontextprotocol/typescript-sdk/commit/b392f02ffcf37c088dbd114fedf25026ec3913d3
• WEBgithub.com/modelcontextprotocol/typescript-sdk/issues/965