CVE-2026-7548 Overview
CVE-2026-7548 is a command injection vulnerability in the Totolink NR1800X router running firmware version 9.1.0u.6279_B20210910. The flaw resides in the sub_41A68C function within /cgi-bin/cstecgi.cgi. Attackers manipulate the setUssd argument to inject arbitrary operating system commands. The vulnerability is exploitable remotely over the network and a public exploit is available. The weakness is classified under [CWE-74] (Improper Neutralization of Special Elements in Output Used by a Downstream Component). The Exploit Prediction Scoring System (EPSS) rates this CVE at the 81.478 percentile, indicating elevated likelihood of exploitation activity.
Critical Impact
Remote attackers with low privileges can execute arbitrary commands on affected Totolink NR1800X devices, leading to full router compromise, traffic interception, and lateral movement into connected networks.
Affected Products
- Totolink NR1800X router
- Firmware version 9.1.0u.6279_B20210910
- Endpoint /cgi-bin/cstecgi.cgi (function sub_41A68C)
Discovery Timeline
- 2026-05-01 - CVE-2026-7548 published to NVD
- 2026-05-01 - Last updated in NVD database
Technical Details for CVE-2026-7548
Vulnerability Analysis
The vulnerability exists in the CGI handler cstecgi.cgi, which processes router configuration requests. The function sub_41A68C handles the setUssd action used for sending Unstructured Supplementary Service Data (USSD) commands through the device cellular interface. User-controlled input from the setUssd parameter reaches a shell execution call without proper sanitization. Attackers append shell metacharacters to the parameter value, breaking out of the intended command context. The injected payload runs with the privileges of the web server process, which on most Totolink devices is root.
Root Cause
The root cause is improper neutralization of special elements passed to a downstream OS command interpreter [CWE-74]. The sub_41A68C function concatenates attacker-controlled string data into a system command without escaping shell metacharacters such as backticks, semicolons, and pipes. No allow-list validation enforces the expected USSD code format before invocation.
Attack Vector
The attack vector is network-based and requires low privileges on the device. An authenticated attacker, or one who reaches the management interface through default or weak credentials, sends a crafted HTTP POST request to /cgi-bin/cstecgi.cgi with the setUssd parameter containing shell metacharacters. The request triggers immediate command execution. Public exploit details are documented in the GitHub Command Injection Analysis and VulDB #360358.
Detection Methods for CVE-2026-7548
Indicators of Compromise
- HTTP POST requests to /cgi-bin/cstecgi.cgi containing the setUssd parameter with shell metacharacters such as ;, |, &, or backticks.
- Unexpected outbound connections originating from the router to attacker-controlled infrastructure.
- New or modified files in writable router paths such as /tmp or /var, including dropped binaries or reverse shell scripts.
- Unusual processes spawned as children of the cstecgi.cgi web handler.
Detection Strategies
- Inspect web server access logs on the router for POST requests to cstecgi.cgi referencing setUssd with non-numeric payloads.
- Deploy network intrusion detection signatures matching command injection patterns in HTTP requests targeting Totolink CGI endpoints.
- Correlate router management plane traffic with threat intelligence feeds tracking VulDB submission #804417.
Monitoring Recommendations
- Monitor egress traffic from router management interfaces for unexpected outbound shells or DNS exfiltration.
- Aggregate router syslog data into a centralized SIEM and alert on authentication anomalies preceding setUssd requests.
- Track firmware version inventory across deployed Totolink devices to identify unpatched units running 9.1.0u.6279_B20210910.
How to Mitigate CVE-2026-7548
Immediate Actions Required
- Restrict access to the router web management interface to trusted management VLANs only and block WAN-side administration.
- Rotate all router administrative credentials and disable any default accounts on Totolink NR1800X devices.
- Audit the device for signs of compromise, including unauthorized configuration changes, foreign processes, and persistence on writable filesystems.
- Replace affected devices with vendor-supported models if no patched firmware becomes available from Totolink.
Patch Information
No vendor patch is referenced in the available CVE data. Consult the Totolink Official Website for firmware updates addressing the setUssd command injection in sub_41A68C. Until a fix ships, treat affected NR1800X units as vulnerable and apply network-level compensating controls.
Workarounds
- Place affected routers behind an upstream firewall that blocks inbound HTTP/HTTPS to the management interface from untrusted networks.
- Disable remote management on the WAN interface through the router configuration menu.
- Segment IoT and consumer router devices onto isolated network zones with strict egress filtering to limit blast radius if exploitation occurs.
# Example iptables rules to restrict router management access to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 192.0.2.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.0.2.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
