CVE-2026-40854 in T-Mobile 5G Box IDU
Summary
by MITRE • 09/16/2026
WNC T-Mobile 5G Box IDU router contains an authentication bypass vulnerability in the portal.cgi component. The session verification mechanism improperly validates the sessionid cookie by checking for the existence of a corresponding file in /tmp/login_user. An attacker can bypass authentication by using directory entries such as "." or ".." in the cookie, allowing unauthorized access to the administration panel.This issue has been fixed in firmware version 1.1.0.651412
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the WNC T-Mobile 5G Box IDU router represents a critical authentication bypass flaw located within the portal.cgi component of its web management interface. This security defect stems from an improper implementation of session verification mechanisms, specifically regarding how the application handles and validates the sessionid cookie provided by clients during HTTP requests. The underlying logic relies on checking for the existence of a corresponding file in the /tmp/login_user directory to determine if a user is authenticated. However, this validation process fails to adequately sanitize or restrict input values before they are used as part of the filesystem path construction. This lack of rigorous input validation creates an opportunity for attackers to manipulate the session identifier string to interfere with standard operating system path resolution behaviors.
By exploiting this flaw, an attacker can bypass authentication controls by injecting directory traversal sequences such as dot-dot-slash (.. or ..) into the sessionid cookie value. In many Unix-like file systems, these special entries refer to parent directories and current working directories respectively. When the vulnerable application constructs a path using unsanitized input from the cookie without proper normalization, it may inadvertently point to a location outside the intended /tmp/login_user scope or trigger unexpected behavior in the file existence check. For instance, if the logic simply checks for any existing file that matches a pattern derived from the manipulated session ID, an attacker might craft a payload that causes the system to find a valid-looking entry elsewhere on the filesystem or exploit race conditions and symlink behaviors associated with temporary directories. This allows unauthorized entities to gain access to the administration panel without providing legitimate credentials, effectively nullifying the primary defense mechanism of the device's management interface.
The operational impact of this vulnerability is severe due to the nature of Internet of Things (IoT) devices like 5G routers which often serve as gateways between private networks and public internet services. Unauthorized access to the administration panel grants an attacker full control over the router configuration. This can lead to a range of malicious activities including reconfiguring network settings, redirecting traffic through man-in-the-middle attacks, disabling security features such as firewalls or encryption protocols, installing persistent malware backdoors, or using the device as part of a botnet for distributed denial-of-service (DDoS) attacks. Since these devices are typically deployed in residential and small business environments where users may not have advanced technical knowledge to detect subtle configuration changes, the risk of prolonged undetected compromise is high. The ability to bypass authentication entirely removes any audit trail or access control logs that might otherwise alert administrators to intrusion attempts.
From a classification perspective, this vulnerability aligns with CWE-287 Improper Authentication and CWE-22 Path Traversal: '..' Sequence in the context of how input is processed by the file system logic. In terms of adversary tactics, it corresponds to MITRE ATT&CK techniques related to Initial Access via Valid Accounts or Exploitation for Credential Access, specifically leveraging misconfigurations rather than stolen credentials. The root cause lies in insufficient validation of user-supplied data before its use in dynamic file path generation, a common pitfall in embedded web application development where resource constraints sometimes lead developers to implement lightweight but insecure session management schemes.
To mitigate this risk, the vendor has released firmware version 1.1.0.651412 which addresses the underlying code flaw by implementing proper input sanitization and path normalization within the portal.cgi component. Administrators should immediately update their devices to this patched version or later if available. In addition to applying software updates, organizations and individuals should enforce strong administrative passwords on all network infrastructure components. It is also advisable to restrict access to the management interface where possible by limiting it to trusted IP addresses via firewall rules rather than exposing it directly to untrusted networks. Regular monitoring of device logs for unusual login attempts or configuration changes can help detect exploitation efforts early, although relying solely on detection without patching leaves a significant window of exposure given the ease with which this specific bypass can be automated and executed remotely by any actor capable of sending HTTP requests to the router's web interface.