CVE-2026-40854: Session auth bypass via cookie value in T-Mobile 5G Box IDU routers

Severity: high (CVSS 8.7 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-16 · updated 2026-09-28
Weakness: CWE-290

Description

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

Affected products

WNC T-Mobile 5G Box IDU: before 1.1.0.651412 — fixed in 1.1.0.651412

How to fix

Upgrade to a fixed version: WNC T-Mobile 5G Box IDU 1.1.0.651412. Check the vendor advisory for mitigations.

References

https://cert.pl/posts/2026/09/CVE-2026-40854

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