CVE-2026-31177: Totolink A3300r Firmware RCE Vulnerability

CVE-2026-31177 Overview

CVE-2026-31177 is a command injection vulnerability in ToToLink A3300R firmware version 17.0.0cu.557_B20221024. The flaw resides in the /cgi-bin/cstecgi.cgi endpoint, which fails to sanitize the stunMinAlive parameter before passing it to a system shell. Unauthenticated remote attackers can inject arbitrary operating system commands through crafted HTTP requests. Successful exploitation grants full command execution on the affected router with the privileges of the web service, typically root. The weakness is tracked under CWE-78 (OS Command Injection).

Critical Impact
Unauthenticated attackers reachable over the network can execute arbitrary commands on the router, leading to full device compromise, traffic interception, and pivoting into internal networks.

Affected Products

ToToLink A3300R router hardware
ToToLink A3300R firmware v17.0.0cu.557_B20221024
Deployments exposing the web management interface (/cgi-bin/cstecgi.cgi) to untrusted networks

Discovery Timeline

2026-04-23 - CVE-2026-31177 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31177

Vulnerability Analysis

The ToToLink A3300R exposes a CGI binary at /cgi-bin/cstecgi.cgi that handles configuration requests for several network services, including STUN keep-alive parameters. The stunMinAlive parameter accepted by this handler is concatenated into a shell command without input validation or escaping. Attackers append shell metacharacters such as ;, |, or backticks to break out of the intended command context. Because the CGI process runs with elevated privileges on the device, injected commands execute with the same level of access.

The attack requires no authentication and no user interaction, and the management interface is reachable over the network. An EPSS score of 0.414% places this issue in the 61st percentile for near-term exploitation likelihood, and a public proof of concept is available in the referenced GitHub repository.

Root Cause

The firmware constructs operating system commands by concatenating attacker-controlled HTTP parameter values directly into shell strings. The stunMinAlive field is expected to be a numeric keep-alive interval, but the handler does not enforce a type check, length limit, or character allowlist. This is a textbook instance of CWE-78, where missing neutralization of special elements in OS commands allows injection.

Attack Vector

An attacker sends a single crafted HTTP POST request to /cgi-bin/cstecgi.cgi containing a malicious stunMinAlive value. The injected payload is appended to the underlying shell command and executed by the router. Details and a working proof of concept are documented in the GitHub PoC Repository.

No sanitized exploit code is reproduced here. Refer to the linked repository for the request structure and parameter format used to trigger command execution.

Detection Methods for CVE-2026-31177

Indicators of Compromise

HTTP POST requests to /cgi-bin/cstecgi.cgi containing shell metacharacters (;, |, &, backticks, $()) in the stunMinAlive field
Unexpected outbound connections from the router to attacker-controlled hosts following management interface traffic
New or modified processes on the device executing shell utilities such as wget, curl, tftp, nc, or sh spawned by the CGI process
Configuration changes to DNS, firewall, or remote administration settings without a corresponding administrator action

Detection Strategies

Inspect HTTP request bodies destined for /cgi-bin/cstecgi.cgi and alert on non-numeric values in the stunMinAlive parameter
Deploy network intrusion detection signatures that flag command injection metacharacters in TOTOLINK CGI parameters
Correlate router management traffic with subsequent outbound connections from the same device to identify post-exploitation callbacks

Monitoring Recommendations

Forward router syslog and HTTP access logs to a centralized log platform for retention and analysis
Baseline expected management interface clients and alert on access from unfamiliar source IPs
Monitor for firmware version 17.0.0cu.557_B20221024 across asset inventories to track exposure

How to Mitigate CVE-2026-31177

Immediate Actions Required

Restrict access to the router web management interface to trusted internal hosts and disable WAN-side administration
Place affected A3300R devices behind a firewall and block inbound HTTP and HTTPS traffic from untrusted networks
Rotate administrative credentials and review router configuration for unauthorized changes
Inventory all A3300R devices running firmware 17.0.0cu.557_B20221024 and prioritize them for replacement or isolation

Patch Information

No vendor advisory or fixed firmware release was referenced in the NVD entry at the time of publication. Operators should monitor the ToToLink support site for updated firmware and apply any released patch immediately. Until a fix is available, treat the device as exposed and rely on compensating controls.

Workarounds

Disable remote management features that expose /cgi-bin/cstecgi.cgi to external interfaces
Segment the router management VLAN from user and guest networks to limit lateral movement
Replace end-of-support or unpatched A3300R units with hardware that receives active security maintenance

bash

# Configuration example: block external access to the router management interface
# Apply on an upstream firewall (iptables syntax)
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -i <wan_iface> -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -i <wan_iface> -j DROP