CVE-2026-0621 is a high-severity vulnerability affecting the MCP TypeScript SDK from lfprojects. The vulnerability allows for regular expression denial of service (ReDoS) due to improperly handled URI patterns. This vulnerability is particularly critical as it can lead to excessive CPU consumption, making the Node.js process unresponsive when specifically crafted malicious URIs are supplied by an attacker.
The vulnerability was published on January 5, 2026, and is classified with a CVSS score of 8.7, indicating a high severity level. Organizations utilizing versions of the MCP TypeScript SDK up to and including 1.25.1 should be aware of the potential impact, which includes denial of service scenarios.
Given the nature of this vulnerability and its potential for exploitation, organizations should prioritize patching immediately. The timeframe for remediation is critical, as the attack vector is network-based with low complexity, meaning that it is relatively easy for attackers to exploit this vulnerability.
Mitigation strategies should include upgrading to a patched version of the SDK as soon as possible, along with implementing monitoring solutions to detect anomalies in URI processing.
Vulnerability Details
The CVE-2026-0621 vulnerability allows for ReDoS in the UriTemplate class of the MCP TypeScript SDK when processing RFC 6570 exploded array patterns. The vulnerability arises from the use of dynamically generated regular expressions that contain nested quantifiers, which can lead to catastrophic backtracking under certain conditions.
The CVSS score of 8.7 reflects the high impact on availability, as exploitation can render the service unresponsive. The vulnerability is categorized under CWE-1333, which indicates issues related to regular expression denial of service.
The affected product is the MCP TypeScript SDK, with all versions prior to the vendor patch being vulnerable. The vulnerability was disclosed on January 5, 2026.
Technical Analysis
The root cause of this vulnerability stems from the handling of URI patterns within the SDK. Specifically, the regular expressions used for URI matching do not effectively manage nested quantifiers, leading to performance degradation when faced with crafted input that exploits this weakness.
The attack vector for this vulnerability is network-based, allowing remote attackers to trigger the ReDoS condition without needing physical access to the system. The attack complexity is low, which means that minimal effort can result in significant service disruption.
No user interaction is required for the exploitation of this vulnerability, thus increasing the likelihood of successful attacks. The impact on availability is classified as high, as successful exploitation can lead to prolonged outages of the affected service.
Risk & Impact Analysis
Risk to organizations includes potential service outages caused by the denial of service vulnerability. The ability for attackers to disrupt services through crafted URIs poses a significant threat, especially for organizations relying on the MCP TypeScript SDK for critical applications.
The blast radius of this vulnerability can extend to any service utilizing the affected SDK, impacting multiple users and potentially leading to a loss of revenue and reputation for organizations. The urgency of addressing this vulnerability is high, as organizations should prioritize patching immediately.
Given the CVSS score of 8.7, the potential for exploitation, and the lack of a known exploit or public proof of concept, organizations are strongly advised to take immediate action. The vulnerability's presence in the wild is currently unconfirmed, but proactive remediation is essential.
Exploitation Status
Signal | Status |
|---|---|
Known Exploit | No |
Public PoC | No |
Actively Exploited | No |
Ransomware Use | No |
Affected Versions
The MCP TypeScript SDK versions up to and including 1.25.1 are affected by this vulnerability. Organizations should ensure that they upgrade to the latest version to mitigate the risk.
Mitigation & Remediation
Organizations must prioritize patching by upgrading the MCP TypeScript SDK to a version that addresses this vulnerability. If immediate upgrades are not possible, organizations should implement workarounds that prevent the processing of malicious URIs.
Configuration hardening and network controls should be established to limit exposure to potential exploitation. Regular monitoring for unusual CPU consumption patterns in service logs may help identify attempts to exploit this vulnerability.
In conclusion, addressing vulnerabilities such as CVE-2026-0621 is critical for maintaining the integrity and availability of software applications.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
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