Stack Overflow in Tenda A18 Router DevName Parameter Enables RCE

Overview

CVE-2023-50585 represents a critical stack overflow vulnerability in the Tenda A18 wireless router firmware version 15.13.07.09 and potentially earlier releases. The vulnerability exists within the formSetDeviceName function, which processes user-supplied input via the devName parameter without proper bounds checking. This classic memory safety flaw enables unauthenticated attackers to overflow the stack buffer, corrupt critical control flow data, and achieve arbitrary code execution with device privileges.

Tenda A18 routers are widely deployed in small office/home office (SOHO) environments globally, making this vulnerability particularly impactful from an attack surface perspective. The router’s web administration interface exposes the vulnerable function to remote attackers without requiring authentication, elevating the threat level to critical infrastructure risk. Security researchers identified this vulnerability through firmware binary analysis and dynamic testing, highlighting the persistent challenges in embedded device security practices within the IoT sector.

Technical Analysis

The vulnerability manifests in the formSetDeviceName function, which handles HTTP requests to set the router’s device name. The function allocates a fixed-size stack buffer to store the device name parameter but fails to validate the input length before copying user-supplied data into this buffer.

Vulnerable Code Pattern

void formSetDeviceName(struct httpRequest *req) {
    char devName[32];  // Fixed 32-byte stack buffer
    
    // Vulnerable: No length validation on user input
    strcpy(devName, req->params["devName"]);
    
    // Process device name
    setDeviceName(devName);
    sendHttpResponse(req, 200, "OK");
}

An attacker can craft an HTTP POST request with a devName parameter exceeding 32 bytes, causing the strcpy function to write beyond the allocated buffer boundaries. This overflow corrupts adjacent stack memory, including function return addresses, saved registers, and other critical data structures.

Exploitation Technique

The stack layout on typical embedded ARM systems places the return address immediately above local variables. By carefully crafting a payload that exceeds 32 bytes with strategically positioned machine code, attackers can:

1. Overflow the 32-byte buffer with arbitrary data
2. Overwrite the saved return address with an address pointing to attacker-controlled code
3. Redirect code execution to shellcode on the stack or existing gadgets in the firmware binary
4. Escalate privileges to root-level device control

Example malicious payload structure:

[32 bytes of filler data] + [4-byte ROP gadget address] + [shellcode/ROP chain]

The lack of stack canaries, Address Space Layout Randomization (ASLR), or Data Execution Prevention (DEP) in the Tenda A18 firmware further simplifies exploitation. These security mitigations are often omitted from IoT devices due to performance constraints and legacy development practices.

Impact

The consequences of CVE-2023-50585 extend far beyond theoretical risk:

• Remote Code Execution: Unauthenticated attackers gain arbitrary command execution as root
• Device Compromise: Full administrative control over router functions, DNS settings, traffic routing, and firewall rules
• Network Infiltration: Compromised routers become pivot points for lateral movement into connected networks
• Persistent Backdoors: Attackers can install firmware modifications or rootkits surviving reboot cycles
• Man-in-the-Middle Attacks: SSL/TLS traffic interception, credential harvesting, and malware distribution
• Data Exfiltration: Access to connected devices, file systems, and sensitive corporate or personal data

This vulnerability affects business continuity for organizations deploying Tenda A18 devices as network edge equipment, particularly in retail, hospitality, and small enterprise verticals.

How to Fix It

Immediate Mitigation

1. Isolate affected devices from production networks until firmware updates are available
2. Disable remote management via web interface; restrict access to trusted internal networks only
3. Network segmentation: Place routers behind additional firewalls limiting access to administration ports (TCP 80/443)

Permanent Remediation

Firmware Update: Contact Tenda support for patched firmware versions addressing CVE-2023-50585. Firmware versions released after January 2024 should contain fixes.

Patched Code Implementation:

#include <string.h>

#define MAX_DEVNAME_LEN 31  // Account for null terminator

void formSetDeviceName(struct httpRequest *req) {
    char devName[32];
    const char *userInput = req->params["devName"];
    
    // FIXED: Validate input length before copying
    if (userInput == NULL || strlen(userInput) > MAX_DEVNAME_LEN) {
        sendHttpResponse(req, 400, "Invalid device name length");
        return;
    }
    
    // Use safe string copy
    strncpy(devName, userInput, MAX_DEVNAME_LEN);
    devName[MAX_DEVNAME_LEN] = '\0';  // Guarantee null termination
    
    if (setDeviceName(devName) == 0) {
        sendHttpResponse(req, 200, "OK");
    } else {
        sendHttpResponse(req, 500, "Internal error");
    }
}

Compiler Security Flags: Rebuild firmware with stack protection enabled:

gcc -fstack-protector-all -fPIE -fPIC -D_FORTIFY_SOURCE=2 \
    -Wformat -Wformat-security -z relro -z now

Our Take

CVE-2023-50585 exemplifies the systemic memory safety challenges plaguing IoT firmware development. While stack overflows represent well-understood attack vectors documented since the 1990s, embedded device manufacturers continue deploying vulnerable patterns due to time-to-market pressures and inadequate secure coding practices.

This vulnerability is not an isolated incident—it reflects broader industry practices where IoT devices receive minimal security scrutiny compared to enterprise software. Organizations must recognize that network edge devices present asymmetric risk: a single compromised router impacts dozens or hundreds of downstream systems.

From an enterprise security perspective, this CVE underscores why firmware assessment capabilities are non-negotiable in the vulnerability management toolkit. Static analysis tools capable of detecting unsafe string operations in compiled binaries can identify similar issues before deployment.

Detection with SAST

Offensive360’s Static Application Security Testing (SAST) engine detects this vulnerability class through multiple detection patterns:

CWE Signatures:

• CWE-674 (Uncontrolled Recursion): Unbounded buffer copies
• CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer): Buffer overflow conditions
• CWE-120 (Buffer Copy without Checking Size of Input)

Pattern Detection:

• Calls to unsafe functions (strcpy, strcat, sprintf, gets) without preceding length validation
• Fixed-size stack allocations receiving user-controlled data
• Absence of bounds checking before memory copy operations
• HTTP parameter handling that bypasses length restrictions

Firmware-Specific Analysis:

• Binary-level detection of vulnerable string operations
• Call graph analysis identifying unauthenticated entry points to unsafe functions
• Stack frame analysis revealing insufficient buffer sizes relative to input sources

Offensive360 flags this vulnerability class across C/C++ codebases and compiled IoT firmware, enabling organizations to eliminate these defects before production deployment.

References

• NVS CVE-2023-50585 Record
• IoT-vulnerable GitHub Repository - Tenda A18 formSetDeviceName Analysis
• CWE-120: Buffer Copy without Checking Size of Input
• CWE-674: Uncontrolled Recursion
• Tenda Official Security Advisory