Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-4253 is a stored OS command injection vulnerability in the Tenda AC8 router firmware version 16.03.50.11. It occurs in the web interface component, specifically in the function route_set_user_policy_rule within the /cgi-bin/UploadCfg endpoint.'}, {'type': 'paragraph', 'content': 'The vulnerability arises because the router processes user-defined policy rules from its configuration without sanitizing special shell characters. The argument wans.policy.list1, which contains policy rules, is parsed and directly used in a system command to configure iptables. Because no sanitization is performed, an attacker can inject shell command substitutions (e.g., $(telnetd)) into the destination IP field of a policy rule.'}, {'type': 'paragraph', 'content': 'When the device reboots and processes the malicious configuration, the injected shell commands are executed with root privileges, allowing the attacker to run arbitrary commands on the device remotely.'}, {'type': 'paragraph', 'content': "The attack requires authentication to the router's web interface to upload a crafted configuration file, but once done, it results in persistent remote root shell access via a telnet daemon started on port 23."}] [1, 2, 3]

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Impact Analysis

[{'type': 'paragraph', 'content': "This vulnerability allows an attacker with administrative access to the router's web interface to execute arbitrary operating system commands as root on the device."}, {'type': 'paragraph', 'content': 'The attacker can gain persistent root shell access remotely by injecting malicious configuration rules that launch a telnet daemon on the router, bypassing normal authentication controls.'}, {'type': 'paragraph', 'content': 'Such control compromises the confidentiality, integrity, and availability of the affected system, potentially allowing the attacker to intercept, modify, or disrupt network traffic, install malware, or use the device as a foothold for further attacks.'}, {'type': 'paragraph', 'content': 'Because the malicious configuration persists across reboots, the attacker maintains long-term control until a factory reset is performed.'}] [1, 2, 3]

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by checking for the presence of a telnet daemon running on port 23 on the Tenda AC8 router, which is unusual for this device and indicates exploitation.'}, {'type': 'paragraph', 'content': "You can also verify if the router configuration contains malicious policy rules by downloading the current configuration file via the router's web interface endpoint `/cgi-bin/DownloadCfg` and inspecting the `wans.policy.list1` field for suspicious command substitution patterns such as `$(telnetd)`."}, {'type': 'paragraph', 'content': 'Suggested commands to detect exploitation include:'}, {'type': 'list_item', 'content': "Use network scanning tools to check if port 23 (telnet) is open on the router's IP address, e.g., `nmap -p 23 <router_ip>`."}, {'type': 'list_item', 'content': 'If you have shell access to the router, check running processes for telnet daemon: `ps | grep telnetd`.'}, {'type': 'list_item', 'content': 'Download and inspect the router configuration file by authenticating to the web interface and accessing `/cgi-bin/DownloadCfg`, then search for `wans.policy.list1` entries containing shell command injection patterns like `$()`.'}] [1, 3]

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Mitigation Strategies

[{'type': 'paragraph', 'content': 'Immediate mitigation steps include disabling the processing of user policy rules by setting `wans.policy.enable` to 0 if possible, to prevent the vulnerable function from executing injected commands.'}, {'type': 'paragraph', 'content': 'Remove or reset any malicious configuration entries in the router configuration, especially those in `wans.policy.list1` that contain command injection payloads.'}, {'type': 'paragraph', 'content': 'Perform a factory reset of the router to clear any persistent malicious configuration stored in flash memory, as the injected commands persist across reboots.'}, {'type': 'paragraph', 'content': "Change the router's admin password to a strong, unique password to prevent unauthorized access."}, {'type': 'paragraph', 'content': 'Monitor the router for unexpected open ports such as telnet on port 23 and disable or block such services if detected.'}, {'type': 'paragraph', 'content': 'Since no official vendor patch or mitigation is currently available, consider replacing the affected device with a secure alternative.'}] [2, 3]

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