CVE-2021-35034 in NBG6604
Summary
by MITRE • 12/29/2021
An insufficient session expiration vulnerability in the CGI program of the Zyxel NBG6604 firmware could allow a remote attacker to access the device if the correct token can be intercepted.
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Analysis
by VulDB Data Team • 12/31/2021
The vulnerability identified as CVE-2021-35034 represents a critical session management flaw within the CGI program of Zyxel NBG6604 firmware versions affected by this issue. This weakness stems from inadequate session expiration mechanisms that fail to properly terminate user sessions after predetermined time intervals or upon logout events. The vulnerability specifically affects the web-based administration interface of the network gateway device, creating a persistent security risk that can be exploited by remote attackers without requiring physical access or elevated privileges. The flaw exists in the implementation of session tokens within the CGI components that handle administrative requests, where session identifiers are not properly invalidated or timed out according to industry best practices for web application security.
The technical exploitation of this vulnerability requires an attacker to intercept a valid session token through network traffic monitoring or other means of token interception. Once obtained, the attacker can leverage this token to maintain unauthorized access to the administrative interface of the Zyxel NBG6604 device for an extended period beyond the intended session lifetime. This persistent access allows the attacker to perform administrative functions including but not limited to configuration changes, firmware updates, network modifications, and potentially full device control. The vulnerability is classified under CWE-613 as "Insufficient Session Expiration" which directly maps to the weakness described in the CVE, where session management controls fail to properly handle session lifecycle events. The flaw demonstrates a fundamental weakness in the device's authentication and authorization framework, where session tokens remain valid indefinitely until manually destroyed or the device is rebooted.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass potential network compromise and complete device takeover. An attacker with access to the administrative interface can modify firewall rules, configure port forwarding, change administrative credentials, disable security features, and potentially establish backdoors within the network infrastructure. This vulnerability particularly affects enterprise and home network environments where the Zyxel NBG6604 serves as a primary gateway device, potentially allowing attackers to gain persistent access to internal networks and compromise multiple connected devices. The remote nature of the attack means that adversaries can exploit this vulnerability from anywhere on the internet, making it particularly dangerous for devices with public internet connectivity. According to ATT&CK framework, this vulnerability maps to T1078.004 "Valid Accounts: SSH Keys" and T1021.001 "Remote Services: Remote Desktop Protocol" through the exploitation of administrative access, though the specific vector here is web-based session manipulation rather than direct protocol exploitation.
Mitigation strategies for CVE-2021-35034 should focus on immediate firmware updates from Zyxel to address the session expiration flaw, as well as implementing network-level controls to monitor and restrict access to administrative interfaces. Organizations should enforce strict access controls by limiting administrative access to trusted networks only, implementing multi-factor authentication where possible, and establishing robust network monitoring to detect unusual traffic patterns or unauthorized access attempts. Network segmentation should be employed to isolate administrative interfaces from general network traffic, while regular security audits should verify that session management controls are properly implemented and functioning. Additionally, implementing intrusion detection systems that can identify and alert on suspicious session token usage patterns will provide enhanced monitoring capabilities for detecting potential exploitation attempts. The vulnerability highlights the importance of proper session management implementation in embedded network devices and underscores the need for regular security assessments of network infrastructure components to prevent persistent access vulnerabilities that could compromise entire network environments.