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High, CVSS 8.7amended

Anthropic's MCP TypeScript SDK has a ReDoS vulnerability

CVE-2026-0621GHSA-8r9q-7v3j-jr4g

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@modelcontextprotocol/sdk 1.25.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
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 · score source
EPSS
  • CVE-2026-0621: 0.44% estimated exploitation activity in the next 30 days; percentile 38.15 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-8r9q-7v3j-jr4g

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-8r9q-7v3j-jr4g

Agent brief

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# Patch Next advisory brief: CVE-2026-0621

Advisory: Anthropic's MCP TypeScript SDK has a ReDoS vulnerability
IDs: CVE-2026-0621, GHSA-8r9q-7v3j-jr4g
Record: https://patchnext.com/advisories/cve-2026-0621 (revision 3, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @modelcontextprotocol/sdk 1.25.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @modelcontextprotocol/sdk: affected >= 1.3.0, < 1.25.2; fixed in 1.25.2

## Suggested inspection (read-only)
- `npm ls @modelcontextprotocol/sdk --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @modelcontextprotocol/sdk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-8r9q-7v3j-jr4g (OSV record GHSA-8r9q-7v3j-jr4g lists affected ranges: npm @modelcontextprotocol/sdk >=1.3.0 <1.25.2 (fixed))
- Verified: https://github.com/advisories/GHSA-8r9q-7v3j-jr4g (GitHub-reviewed advisory GHSA-8r9q-7v3j-jr4g lists affected ranges: npm @modelcontextprotocol/sdk >=1.3.0 <1.25.2)

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