CVE-2026-0621: @modelcontextprotocol/sdk Denial of Service Vulnerability
HIGHCVE-2026-0621 is a HIGH denial of service vulnerability (CWE-1333: Inefficient Regular Expression Complexity) affecting @modelcontextprotocol/sdk libraries in the npm ecosystem. The issue involves anthropic's mcp typescript sdk has a redos vulnerability. Remediation is available by upgrading affected packages to version 1.25.2.
CVE-2026-0621 at a Glance
How to Fix CVE-2026-0621
To remediate this vulnerability, upgrade all affected @modelcontextprotocol/sdk dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
npm install @modelcontextprotocol/sdk@1.25.2Run the following command in your project directory to verify that all resolved dependencies use the patched release:
npm ls @modelcontextprotocol/sdkHow to Check if You Are Affected
To determine whether your Node.js application is affected by CVE-2026-0621, inspect your project dependencies using npm:
npm ls @modelcontextprotocol/sdkAnalyze the terminal output to locate any version of @modelcontextprotocol/sdk in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.25.2), your application is affected.
Note: The npm ls @modelcontextprotocol/sdk command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-1333: Inefficient Regular Expression Complexity, where the product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential. In @modelcontextprotocol/sdk, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
CVE-2026-0621 Affected @modelcontextprotocol/sdk Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
@modelcontextprotocol/sdk
|
npm |
≥ 1.3.0
< 1.25.2
|
1.25.2
|
References
Frequently Asked Questions
What is CVE-2026-0621?
Which @modelcontextprotocol/sdk versions are affected?
What version fixes CVE-2026-0621?
Is @modelcontextprotocol/sdk affected?
What CWE is associated with CVE-2026-0621?
How can I check npm dependencies for CVE-2026-0621?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-8r9q-7v3j-jr4g",
"modified" : "2026-07-15T18:32:03Z",
"published" : "2026-01-05T21:30:33Z",
"aliases" : [ "CVE-2026-0621" ],
"summary" : "Anthropic's MCP TypeScript SDK has a ReDoS vulnerability",
"details" : "### Impact\n\nA 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.\n\n**Who is affected:** MCP servers that register resource templates with exploded array patterns and accept requests from untrusted clients.\n\n**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.\n\n### Affected Versions\n\nAll versions of `@modelcontextprotocol/sdk` prior to the patched release.\n\n### Patches\n\nv1.25.2 contains b392f02ffcf37c088dbd114fedf25026ec3913d3 the fix modifies the regex pattern to prevent backtracking.\n\n### Workarounds\n\n- Avoid using exploded patterns (`{/id*}`, `{?tags*}`) in resource templates\n- Implement request timeouts and rate limiting\n- Validate URIs before processing to reject suspicious patterns",
"severity" : [ {
"type" : "CVSS_V4",
"score" : "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"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "npm",
"name" : "@modelcontextprotocol/sdk"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "1.3.0"
}, {
"fixed" : "1.25.2"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk/security/advisories/GHSA-cqwc-fm46-7fff"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-0621"
}, {
"type" : "WEB",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk/issues/965"
}, {
"type" : "WEB",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk/commit/7f0cf730"
}, {
"type" : "WEB",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk/commit/b392f02ffcf37c088dbd114fedf25026ec3913d3"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk"
}, {
"type" : "WEB",
"url" : "https://github.com/modelcontextprotocol/typescript-sdk/releases/tag/v1.25.2"
}, {
"type" : "WEB",
"url" : "https://www.vulncheck.com/advisories/mcp-typescript-sdk-uritemplate-exploded-array-pattern-redos"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-1333" ],
"severity" : "HIGH",
"github_reviewed" : true,
"github_reviewed_at" : "2026-01-06T21:20:37Z",
"nvd_published_at" : "2026-01-05T21:16:14Z"
}
}