CVE-2026-0621Public exploitNetwork attack vector

Service hang via exploded-template regex backtracking

Published Jan 5, 2026 · Updated Jan 5, 2026

ReDoS in MCP TypeScript SDK 1.25.1 and earlier allows remote attackers to hang MCP servers with crafted resource URIs. UriTemplate.partToRegExp generates a nested-quantifier pattern for exploded variables, and match applies it to the client-controlled resources/read URI. Exposure requires a server to register an exploded resource template and accept requests from untrusted MCP clients; successful input monopolizes CPU and interrupts service for all clients.

CVSS severity8.7
High
EPSS probability0.45%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

ReDoS in MCP TypeScript SDK 1.25.1 and earlier allows remote attackers to hang MCP servers with crafted resource URIs. UriTemplate.partToRegExp generates a nested-quantifier pattern for exploded variables, and match applies it to the client-controlled resources/read URI. Exposure requires a server to register an exploded resource template and accept requests from untrusted MCP clients; successful input monopolizes CPU and interrupts service for all clients.

The record

CVE
CVE-2026-0621
Published
Jan 5, 2026
Updated
Jan 5, 2026
Vendor
Model Context Protocol
Product
MCP TypeScript SDK
Classifications
CWE-1333, CAPEC-492, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 5, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 5, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=1.25.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · MCP TypeScript SDKversion=0 <=1.25.1

Published CVSS scores

8.7VulnCheckCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
CAPEC-492Regular Expression Exponential Blowup
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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