Security Vulnerability Discovered in TL-WR841N Router
/ 3 min read
Quick take - A recent analysis has revealed a command injection vulnerability in early versions of the TL-WR841N router, allowing authenticated attackers to execute arbitrary commands on the device’s operating system through its diagnostics webpage due to insufficient validation of user input.
Fast Facts
- A significant command injection vulnerability has been found in early versions of the TL-WR841N router, affecting its diagnostics webpage.
- Authenticated attackers can execute arbitrary commands on the device’s operating system due to insufficient validation of user input.
- The vulnerability was identified in the firmware version TL-WR841N(EU)_V14_171208, specifically in the function util_execSystem within the file /lib/cmm.so.
- Exploitation can allow attackers to retrieve sensitive files like /etc/passwd and establish reverse shell connections by appending malicious commands.
- To mitigate such vulnerabilities, developers should avoid executing OS commands from high-level languages and implement input sanitization through a whitelist approach.
Security Vulnerability Discovered in TL-WR841N Router
Overview of the Vulnerability
A recent analysis has uncovered a significant security vulnerability in early versions of the TL-WR841N router. This vulnerability specifically affects the router’s diagnostics webpage and has been classified as a command injection flaw. The flaw allows authenticated attackers to execute arbitrary commands on the device’s operating system through its web interface. The issue arises due to insufficient validation of user-supplied input, which is processed by a system shell or command-line interpreter without proper checks.
Testing and Analysis
Testing was conducted using the firmware version TL-WR841N(EU)_V14_171208. During testing, it was discovered that executing commands via the device’s web interface generates a debug log on the device’s UART interface. A detailed examination of the firmware was performed by extracting it using SPI. The core functionality of the system was found to reside in the file /lib/cmm.so. Analysis of this file was conducted using Ghidra software, which identified a particular function, util_execSystem, where user input is passed directly to the system without any validation.
The debug logs demonstrated that certain commands could be executed by the host. Various delimiters available in UNIX systems, such as &, &&, |, ||, and ;, were noted. Notably, commands enclosed in backticks ` are executed before the remaining command line. Exploiting this vulnerability can be achieved by appending malicious commands following backticks and double quotes. To illustrate the potential impact, exploitation testing successfully retrieved the contents of the /etc/passwd file. Furthermore, the exploit can be modified to execute arbitrary system commands, including establishing a reverse shell connection.
Recommendations for Mitigation
The exploitation process requires three distinct HTTP requests to successfully trigger the condition. To mitigate the risk of command injection vulnerabilities, developers are advised to avoid invoking operating system commands from higher-level programming languages. In cases where such functionality is unavoidable, implementing a whitelist approach to sanitize input is essential. This approach enhances the security of the application and protects against potential exploitation.
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