Source: cve@mitre.org
Telnet allows a remote client to specify environment variables including LD_LIBRARY_PATH, allowing an attacker to bypass the normal system libraries and gain root access.
Remote attackers can exploit a vulnerability in Telnet to gain root access by manipulating the LD_LIBRARY_PATH environment variable. This allows attackers to load malicious libraries, bypassing system security and potentially achieving complete system compromise.
Step 1: Connection Establishment: The attacker connects to the vulnerable Telnet server.
Step 2: Environment Variable Injection: The attacker sets the LD_LIBRARY_PATH environment variable in their Telnet session to point to a directory they control, such as /tmp/attacker_libs.
Step 3: Malicious Library Placement: The attacker places a malicious shared library (e.g., libevil.so) in the directory specified by LD_LIBRARY_PATH.
Step 4: Telnet Execution: When Telnet attempts to execute a command or process, it loads shared libraries based on the LD_LIBRARY_PATH environment variable.
Step 5: Library Loading and Code Execution: The system loads the attacker's malicious library instead of the legitimate system library. The malicious library then executes arbitrary code with the privileges of the Telnet process (typically root), granting the attacker control of the system.
The vulnerability stems from Telnet's failure to properly sanitize or restrict the environment variables passed from a client. Specifically, the LD_LIBRARY_PATH variable, which dictates where the dynamic linker searches for shared libraries, is not adequately controlled. By setting LD_LIBRARY_PATH to point to a directory controlled by the attacker, they can force the system to load a malicious shared library. This library, crafted to execute arbitrary code, then runs with the privileges of the Telnet process, which typically runs as root. The core flaw is a lack of input validation and a failure to protect against environment variable manipulation, leading to a privilege escalation vulnerability. The root cause is a design flaw in Telnet's handling of environment variables, not a specific coding error like a buffer overflow or race condition.
While no specific APT groups are exclusively known for exploiting this vulnerability, it's a common technique used by various actors due to its simplicity and effectiveness on vulnerable systems. This vulnerability is not listed in the CISA KEV catalog, likely due to its age and the focus on newer, more prevalent vulnerabilities.
Monitor Telnet server logs for suspicious activity, such as unusual environment variable settings (especially LD_LIBRARY_PATH).
Analyze network traffic for Telnet sessions that set the LD_LIBRARY_PATH variable to non-standard locations.
Inspect system logs for the loading of unexpected shared libraries.
Implement file integrity monitoring to detect changes to system libraries.
Use intrusion detection systems (IDS) with signatures specifically designed to detect this type of attack.
Disable Telnet and use a more secure protocol like SSH for remote access.
If Telnet is required, configure the Telnet server to not allow environment variables to be passed from the client.
Implement strict input validation and sanitization for all user-supplied data, including environment variables.
Regularly update the operating system and all software to patch known vulnerabilities.
Implement least privilege principles, ensuring that the Telnet process runs with the minimum necessary privileges.
Use a host-based intrusion detection system (HIDS) to monitor for malicious activity.
Consider using a network intrusion detection system (NIDS) to detect malicious Telnet traffic.