CVE-2021-32589 - How a Use-After-Free Bug in FortiManager and FortiAnalyzer Can Lead to Remote Code Execution

In mid-2021, cybersecurity professionals were alerted to a critical vulnerability affecting Fortinet’s popular network management products—FortiManager and FortiAnalyzer. Labeled CVE-2021-32589, this issue is particularly dangerous. It allows any attacker on the network to send a malicious request and run arbitrary code as the root user, without even logging in!

Let’s break down what happened, how it works, and walk through the essentials of this exploit—using clear, simple language.

What is CVE-2021-32589?

CVE-2021-32589 is a Use-After-Free (UAF) vulnerability, explained by CWE-416. A use-after-free occurs when a program continues to use a pointer after the memory it points to has already been freed. This can lead to program crashes, data corruption, or—most dangerously—code execution by hijacking freed memory.

In this case, the issue was found in the fgfmsd daemon—a background process running on FortiManager and FortiAnalyzer—that listens for remote management requests on the fgfm port (default is 541, but may vary in your setup).

5.2.10 to 5.2.4

You’ll find this bug documented in the Fortinet PSIRT Advisory, and covered in security databases like CVE Details and NIST NVD.

Technical Breakdown: How the Exploit Works

The fgfmsd daemon is a service responsible for processing incoming management commands sent to your Forti device’s fgfm port. Due to incorrect memory management, specifically around dynamic objects freed at the wrong time, an attacker can send a crafted request that:

Leaves a reference to it elsewhere.

3. Causes the program to use this freed memory—giving an attacker a chance to control what the program reads or executes from it.

If a hacker can control this freed memory (by carefully timing what gets written there), they can hijack the process and execute code with the highest privileges. In FortiManager and FortiAnalyzer, this means root access.

Code Snippet: Simulating a Use-After-Free in C

To help you understand the underlying issue, here’s a basic C code snippet—for illustration only—that shows how a Use-After-Free bug can happen:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    char *buf = malloc(64);
    strcpy(buf, "Sensitive Data");
    free(buf);

    // This is the bug: using 'buf' after it's already freed!
    printf("Data: %s\n", buf);

    // If an attacker gets in here, they could overwrite buf's memory and hijack the flow.
    return ;
}

In real appliance code, a similar misuse can have much greater consequences.

Connect: Open a socket to the fgfm port (often 541).

3. Crafted Request: Send a specially created packet that manipulates the memory allocation/deallocation.
4. Heap Spraying: Repeatedly send specific data to fill freed memory with attacker-controlled code or pointers.

Trigger Execution: Cause the program to execute code from the overwritten memory area.

*Note: Public proof-of-concept code is not widely available due to the seriousness of this bug. Exploit development would require reverse engineering the fgfmsd protocol and some C programming knowledge.*

But—a generic approach using Python sockets might look like this

import socket

host = 'target_ip'
port = 541

malicious_packet = b'\xde\xad\xbe\xef' + b'A' * 100  # Example pattern, not real exploit

s = socket.create_connection((host, port))
s.sendall(malicious_packet)
s.close()

*This code simply opens a connection and sends junk data, but a real exploit would need to trigger the use-after-free condition with extreme precision.*

References

- Fortinet PSIRT Advisory FG-IR-21-085
- NVD CVE-2021-32589
- CVE Details: CVE-2021-32589
- CWE-416: Use After Free

Conclusion

CVE-2021-32589 stands as a sobering reminder that even sophisticated security appliances can hide dangerous vulnerabilities. Use-after-free bugs are especially toxic—they can offer root access to attackers in a flash. If you run a FortiManager or FortiAnalyzer affected by this bug, patch it right away and limit network access to your management ports.

Stay vigilant, stay patched, and always follow security advisories from your vendors!


*This post was written exclusively for your cybersecurity awareness. Always test vulnerabilities and exploits only in your own environments or with explicit permission.*

Timeline

Published on: 12/19/2024 13:15:05 UTC
Last modified on: 01/31/2025 17:42:05 UTC