CVE-2026-0621
Published: 05 January 2026
Summary
CVE-2026-0621 is a high-severity Inefficient Regular Expression Complexity (CWE-1333) vulnerability in Lfprojects Mcp Typescript Sdk. Its CVSS base score is 8.7 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 31.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
This vulnerability is AI-related — categorised as AI Agent Protocols and Integrations; in the Protocol-Specific Risks risk domain.
The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and SC-5 (Denial-of-service Protection).
Deeper analysis
CVE-2026-0621 is a regular expression denial of service (ReDoS) vulnerability affecting Anthropic's MCP TypeScript SDK in versions up to and including 1.25.1. The issue resides in the UriTemplate class during processing of RFC 6570 exploded array patterns, where a dynamically generated regular expression for URI matching includes nested quantifiers. These can trigger catastrophic backtracking on specially crafted inputs, leading to excessive CPU consumption and potential unresponsiveness of the Node.js process.
The vulnerability has a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating network accessibility with low attack complexity, no privileges or user interaction required. Any remote attacker can exploit it by supplying a malicious URI to a vulnerable application using the SDK, causing the Node.js process to become unresponsive and resulting in a denial of service. It is associated with CWE-1333 (Inefficient Regular Expression Complexity).
Advisories detail the issue in GitHub issue #965 at https://github.com/modelcontextprotocol/typescript-sdk/issues/965 and the VulnCheck advisory at https://www.vulncheck.com/advisories/mcp-typescript-sdk-uritemplate-exploded-array-pattern-redos. Practitioners should consult these for mitigation guidance, such as updating to a patched SDK version beyond 1.25.1.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-0800
Vulnerability details
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…
more
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.
- CWE(s)
AI Security AnalysisAI
- AI Category
- AI Agent Protocols and Integrations
- Risk Domain
- Protocol-Specific Risks
- OWASP Top 10 for LLMs 2025
- None mapped
- Classification Reason
- Matched keywords: anthropic, mcp
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
ReDoS vulnerability directly enables application-layer DoS via crafted input triggering catastrophic backtracking and resource exhaustion.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Remediating the ReDoS flaw by updating to a patched SDK version beyond 1.25.1 directly eliminates the vulnerability in the UriTemplate class.
Validating URI inputs prior to processing by the UriTemplate class prevents specially crafted exploded array patterns from triggering catastrophic backtracking.
Implementing denial-of-service protections such as CPU resource limits and request throttling mitigates excessive consumption from ReDoS exploitation.