CVE 2020–29138 — Improper Access Control in the SAGEMCOM router, model F@ST 3486 running NET_4.109.0

Alexandre Vieira
2 min readNov 27, 2020

The SAGEMCOM router, model F@ST3486 NET, running the NET_4.109.0 software version, contains an Improper Access Control vulnerability in the configuration backup functionality. This router is widely distributed by the ISP provider CLARO company, for their customers in Brazil.

The vulnerability occurs when there is a valid session running in the router web interface.

As long as there is any valid session opened, any unauthenticated request to http://<router-ip>/backupsettings.conf will allow the router configuration download.


Access the router’s web interface and log in.

Make a request to the /backupsettings.conf path removing any cookie data.

Request wihtout any session data

Making the request from a different IP than the one which initialized the valid session:

curl request without any session data

The backupsettings.conf file contains sensitive information, including the administration username and password.

If the “Remote Configuration Management” is activated in the router, the access to the backup configuration file becomes available through the WAN to all Internet at the TCP:6080 .

In the image bellow I used a web proxy and accessed the WAN IP address of the router “189.61…” to ensure the external communication.