CVE-2021-27705 in Tenda
Summary
by MITRE • 04/14/2021
Buffer Overflow in Tenda G1 and G3 routers with firmware v15.11.0.17(9502)_CN allows remote attackers to execute arbitrary code via a crafted action/"qosIndex "request. This occurs because the "formQOSRuleDel" function directly passes the parameter "qosIndex" to strcpy without limit.
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Analysis
by VulDB Data Team • 04/21/2021
The vulnerability identified as CVE-2021-27705 represents a critical buffer overflow flaw affecting Tenda G1 and G3 routers running firmware version v15.11.0.17(9502)_CN. This security weakness resides within the router's web interface handling mechanism and specifically targets the formQOSRuleDel function which processes Quality of Service rule deletion requests. The flaw stems from inadequate input validation and parameter sanitization, creating a pathway for remote code execution through carefully crafted network requests.
The technical implementation of this vulnerability demonstrates a classic buffer overflow condition where the qosIndex parameter is directly fed into the strcpy function without any bounds checking or length validation. This primitive function call creates an exploitable condition where an attacker can supply malicious input exceeding the allocated buffer space, leading to memory corruption and potential arbitrary code execution. The vulnerability operates at the application layer and requires no authentication, making it particularly dangerous as it can be exploited remotely from the internet. According to CWE classification, this represents a CWE-121: Stack-based Buffer Overflow, which is a well-documented and severe vulnerability type that has been exploited in numerous network device compromises.
The operational impact of this vulnerability extends beyond simple remote code execution to encompass complete system compromise and potential network infiltration. An attacker who successfully exploits this vulnerability gains the ability to execute arbitrary code on the affected router, potentially allowing them to install backdoors, modify network configurations, redirect traffic, or establish persistent access points within the network. This weakness affects not just individual devices but entire network infrastructures, as compromised routers can serve as entry points for lateral movement throughout the network. The vulnerability's remote exploitability means that attackers do not need physical access to the device or network proximity, making it particularly concerning for enterprise and home network environments.
Mitigation strategies for CVE-2021-27705 should prioritize immediate firmware updates from Tenda, as this is the most effective defense against the identified buffer overflow condition. Network administrators should also implement network segmentation and access controls to limit the potential impact of successful exploitation. The use of intrusion detection systems and network monitoring tools can help detect suspicious traffic patterns associated with exploitation attempts. Additionally, implementing proper input validation and parameter sanitization in web applications aligns with ATT&CK framework techniques for preventing remote code execution, specifically targeting T1203: Exploitation for Client Execution and T1059: Command and Scripting Interpreter. Organizations should also consider disabling unnecessary services and ports, implementing network access control lists, and regularly auditing network device configurations to reduce the attack surface and prevent similar vulnerabilities from being exploited in other network components.