CVE-2026-21874: Medium Vulnerability in zauberzeug NiceGUI

This vulnerability allows unauthenticated attackers to exhaust Redis connections within the NiceGUI framework, impacting service availability. The affected versions range from 2.10.0 to 3.4.1, and the vulnerability has been classified with a medium severity level, scoring 5.3 on the CVSS scale. The risk to organizations includes service degradation as Redis connections are not released due to repeated opening and closing of browser tabs on any NiceGUI application utilizing Redis-backed storage. This condition leads to a denial of service when Redis reaches its connection limit, although the application continues to accept new connections.

The vulnerability was publicly disclosed on January 8, 2026, and patched in version 3.5.0 of the NiceGUI framework. Organizations using affected versions should evaluate their exposure and prioritize patching to mitigate potential impacts. Given the nature of the vulnerability, it carries a risk of service interruption, making it essential for organizations to act promptly.

As of the latest analysis, there are no known exploits or proof-of-concept code publicly available. However, the nature of the vulnerability suggests that it could be leveraged by attackers to disrupt services, underscoring the urgency for organizations to address this issue in their patch management cycles.

Organizations should prioritize patching immediately. Keeping systems updated will help defend against potential service outages and maintain operational integrity.

Vulnerability Details

The vulnerability in question, identified as CVE-2026-21874, affects NiceGUI, a Python-based UI framework developed by zauberzeug. The official CVE description indicates that from versions v2.10.0 to v3.4.1, an unauthenticated attacker can exhaust Redis connections by repeatedly opening and closing browser tabs in applications that utilize Redis for storage. As a result, connections are not released, leading to degraded service when Redis reaches its connection limit. Errors are logged, yet the application remains operational but with compromised storage functionality.

The vulnerability is classified under CWE-772, and its CVSS score of 5.3 indicates a medium severity level. The attack vector is classified as network-based, and the prerequisites for exploitation are minimal, requiring no privileges or user interaction.

The vulnerability was patched in version 3.5.0, published on January 15, 2026.

Technical Analysis

The root cause of this vulnerability stems from the handling of Redis connections within the NiceGUI framework. When an application allows users to repeatedly open and close tabs, it fails to release the connections properly, leading to exhaustion of available Redis connections. This condition can be exploited by attackers to force the application into a state of degraded performance or denial of service.

The attack vector for this vulnerability is network-based, with low complexity. Attackers do not require any privileges or user interaction to exploit this vulnerability, making it particularly concerning. The impact on availability is categorized as low, as the application continues to accept new connections, albeit with compromised functionality.

Risk & Impact Analysis

The real-world deployment risk associated with this vulnerability is significant. Organizations utilizing the NiceGUI framework with affected versions may experience service degradation or outages due to the exhaustion of Redis connections. This can lead to disrupted user experiences and potential loss of business continuity.

The blast radius for this vulnerability may extend to all users of the affected applications, as any user can initiate the exploitation by opening and closing tabs. Given the widespread use of Redis in applications, the implications of this vulnerability are concerning, particularly in high-traffic environments.

Based on the CVSS score, organizations should address this vulnerability in their priority patch cycle. The potential for service disruption necessitates immediate attention from security teams, particularly those responsible for application development and infrastructure management.

Affected Versions

The vulnerability affects all versions of NiceGUI from 2.10.0 up to, but not including, 3.5.0. Organizations using these versions are at risk and should consider upgrading to the patched version 3.5.0 to mitigate the vulnerability.

Mitigation & Remediation

Organizations should prioritize upgrading to NiceGUI version 3.5.0, which contains a patch for this vulnerability. If immediate patching is not feasible, consider implementing workarounds such as limiting the number of concurrent Redis connections or monitoring Redis usage to detect potential abuse. Configuration hardening should also be performed to minimize the attack surface.

Organizations may also benefit from conducting regular security assessments, including penetration testing, to identify and mitigate vulnerabilities in their applications. For more comprehensive security measures, consider utilizing penetration testing services to ensure their applications are secure.

Detection Guidance

To detect potential exploitation of this vulnerability, organizations should monitor logs for unusual patterns of Redis connection usage. Behavioral anomalies, such as a sudden increase in opened and closed connections from the same user or IP address, should be flagged for further investigation. Additionally, network signatures that identify excessive Redis connection attempts can also be useful in identifying potential abuse. Security teams should ensure systems are configured to alert on such anomalies.

AppSecure Threat Intelligence Insight

This vulnerability highlights the importance of robust resource management in web applications. As applications become increasingly complex, ensuring that resources like database connections are managed effectively is critical to preventing service disruptions. Security teams should regularly review their application's architecture and connection management practices to identify potential weaknesses.

In light of this vulnerability, organizations are encouraged to strengthen their security postures by adopting best practices in application security and regularly updating their systems. For further guidance, security teams can refer to resources such as the vulnerability management program design and the penetration testing methodology to ensure comprehensive security coverage.

Overall, organizations should remain vigilant and proactive in their approach to security, continuously assessing and updating their defenses against emerging threats.