CVE-2026-25857 Overview
CVE-2026-25857 is an operating system command injection vulnerability [CWE-78] in the Tenda G300-F router. The flaw resides in the WAN diagnostic handler formSetWanDiag, which builds a shell command invoking curl using attacker-controlled input without proper neutralization. An authenticated attacker with access to the management interface can append shell metacharacters to inject additional commands. Exploitation yields arbitrary command execution with the privileges of the management process on the device. Tenda G300-F firmware versions 16.01.14.2 and earlier are affected.
Critical Impact
Remote attackers with management interface access can execute arbitrary OS commands on affected Tenda G300-F routers, leading to full device compromise and pivoting into the connected network.
Affected Products
- Tenda G300-F router firmware version 16.01.14.2 and prior
- Tenda G300-F hardware revision 1.0
- Management interface exposing the formSetWanDiag endpoint
Discovery Timeline
- 2026-02-07 - CVE-2026-25857 published to NVD
- 2026-03-05 - Last updated in NVD database
Technical Details for CVE-2026-25857
Vulnerability Analysis
The Tenda G300-F web management application exposes a WAN diagnostic feature reachable via the formSetWanDiag form handler. The handler accepts user-supplied parameters and forwards them into a shell invocation of the curl utility. Because the implementation concatenates input directly into the command string, shell metacharacters such as backticks, semicolons, and $() substitutions are interpreted by the underlying shell. This is a textbook OS command injection condition mapped to [CWE-78].
Successful exploitation provides command execution in the context of the router's management process, which on small office and residential gateways typically runs with elevated privileges. From this position an attacker can read configuration secrets, modify firewall and DNS settings, install persistent implants, or pivot to internal hosts. The vulnerability requires network reachability to the management interface and valid administrative credentials.
Root Cause
The root cause is improper neutralization of special elements used in an OS command. The formSetWanDiag routine constructs a curl command line by string concatenation of attacker-controlled fields. No allowlist validation, shell-escape function, or execve-style argument-vector invocation is used, so embedded shell syntax is honored by the spawned shell.
Attack Vector
The attack vector is network-based against the router management interface. An attacker submits a crafted request to the WAN diagnostic endpoint with shell metacharacters embedded in a diagnostic parameter. The server-side handler passes the tainted string to the system shell, which executes both the intended curl command and the injected payload. Technical write-ups describing the specific vulnerable parameter and request layout are available in the VulnCheck Tenda G300-F Advisory and the research write-up by Evan.
Detection Methods for CVE-2026-25857
Indicators of Compromise
- HTTP POST requests to the formSetWanDiag endpoint containing shell metacharacters such as ;, |, &&, backticks, or $(...) in diagnostic parameters
- Unexpected outbound connections from the router to attacker-controlled infrastructure following diagnostic requests
- New or modified processes spawned by the router management daemon outside normal curl activity
- Unauthorized changes to DNS, firewall, or administrative credentials on the device
Detection Strategies
- Inspect web server and management interface logs for formSetWanDiag requests originating from unexpected source addresses
- Apply network intrusion detection signatures matching shell metacharacter patterns in HTTP request bodies targeting Tenda management endpoints
- Correlate authentication events on the router admin account with subsequent diagnostic requests to identify suspicious sequences
Monitoring Recommendations
- Forward router syslog and management access logs to a centralized logging platform for long-term retention and correlation
- Monitor for new outbound connections from router IP addresses to non-standard ports or unfamiliar hosts
- Track firmware version inventory to identify devices still running 16.01.14.2 or earlier
How to Mitigate CVE-2026-25857
Immediate Actions Required
- Restrict access to the router management interface to trusted internal hosts and disable WAN-side administration
- Rotate administrative credentials on all Tenda G300-F devices and enforce strong, unique passwords
- Audit recent management interface activity for unexpected formSetWanDiag requests
- Isolate affected routers from sensitive network segments until a vendor patch is applied
Patch Information
No vendor advisory or fixed firmware version is referenced in the published CVE record at the time of writing. Refer to the Tenda G300 product page for firmware release notes, and consult the VulnCheck advisory for updates on remediation availability.
Workarounds
- Disable remote management on the WAN interface and limit administrative access to a dedicated management VLAN
- Place the router behind an upstream firewall that blocks unsolicited inbound access to administrative HTTP/HTTPS ports
- Segment IoT and infrastructure devices from user and server networks to limit blast radius if the device is compromised
- Replace end-of-support devices that do not receive timely security updates
# Configuration example: block external access to router admin interface
iptables -I INPUT -i wan0 -p tcp --dport 80 -j DROP
iptables -I INPUT -i wan0 -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.
