Vulnerability · Published Sep 16, 2026 · Assigned by CERT-PL

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

High 8.7

In plain English

A high-severity authentication bypass by spoofing in T-Mobile 5G Box IDU from WNC. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).

Severity (CVSS 4.0)8.7High

Exploit odds (30 days)0.3%top 75.0% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 (as reported)

Vendor	Product	Versions
WNC	T-Mobile 5G Box IDU	0 – before 1.1.0.651412

Weakness: CWE-290 · Vector: CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

References

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