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CVE-2026-0621 Anthropic CVE debrief

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. This vulnerability has a high impact on organizations using the affected SDK versions, as it can lead to service disruption and potential data breaches.

Vendor
Anthropic
Product
MCP TypeScript SDK
CVSS
HIGH 8.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-01-05
Original CVE updated
2026-07-14
Advisory published
2026-01-05
Advisory updated
2026-07-14

Who should care

Organizations using Anthropic's MCP TypeScript SDK versions up to and including 1.25.1 should prioritize updating to a patched version to prevent potential denial-of-service attacks. Additionally, security teams and vulnerability management teams should be aware of the potential impact of this vulnerability on their systems and take steps to mitigate it. Operators and administrators of affected systems should also be aware of the potential risks and take steps to protect their systems.

Technical summary

The vulnerability exists in the UriTemplate class of the MCP TypeScript SDK, specifically in the handling of RFC 6570 exploded array patterns. This feature allows for complex URI templating but also introduces the risk of ReDoS when not properly sanitized. The dynamically generated regular expression used for URI matching can enter into catastrophic backtracking with specially crafted inputs, leading to high CPU usage and potential service disruption. The affected SDK versions do not properly validate or sanitize the input URIs, making it possible for attackers to craft malicious inputs that exploit this vulnerability.

Defensive priority

High

Recommended defensive actions

  • Update to a patched version of the MCP TypeScript SDK
  • Implement input validation and sanitization for URI templates
  • Monitor for suspicious URI requests
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-01-05T21:16:14.533Z and was last modified on 2026-07-14T16:16:43.063Z. The NVD entry is currently Analyzed. The vulnerability was reported by an external researcher and is based on limited source information. Further verification and testing are recommended to confirm the impact and scope of this vulnerability.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-0621 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-0621

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-0621 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-0621

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.