CVE-2023-29181 - Exploiting Fortinet FortiOS, FortiProxy & FortiPAM via Format String Flaw

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Summary:
CVE-2023-29181 is a critical vulnerability found in multiple versions of Fortinet FortiOS, FortiProxy, and FortiPAM. The flaw is caused by unsafe use of externally-controlled format strings, which lets attackers execute code or commands on affected devices just by sending malicious input.

This long-read explains how the vulnerability works, who is affected, how it might be exploited, includes code snippets, and provides references for further reading. This is exclusive content designed for easy understanding.

What's the Issue?

In several Fortinet product versions, certain command processing functions accept user input and pass it to low-level formatting functions like printf or snprintf without cleaning or validating it. If the input includes format specifiers (like %x, %s, %n), the function will interpret them, not as plain text, but as instructions. This is called an "externally-controlled format string" bug, and it can lead to information disclosure or even remote code execution.

This vulnerability is extremely dangerous—if an attacker sends a specially crafted command, they can execute arbitrary code as the system user running the process.

1.. through 1..3

These versions are deployed in countless enterprises worldwide, protecting networks, proxies, and privileged access.

Let's look at a simplified code snippet illustrating the bug

// BAD: vulnerable code example in C
void handle_command(char* user_input) {
    char buf[256];
    snprintf(buf, sizeof(buf), user_input); // <-- vulnerable!
    printf("%s\n", buf);
}

Here, user_input could be something innocent, like "show status", but if an attacker sends "%x %x %x %x", the function will print out values from the stack, leaking memory contents.

Even worse, with more advanced exploitation, an attacker might write to memory using %n or craft inputs that inject code.

Step-by-step Example

Suppose the vulnerable device is reachable on the network, and you control some command input (like a custom administrative command or debug function):

`

If the output displays something like fd58c20... instead of your text, stack values are leaking—that’s already a problem!

`

- With enough info, a skilled attacker could even inject a ROP chain or shellcode, depending on system memory layout and protections.

Example with Python (for illustration)

import requests

# Change these for your environment
url = "https://vulnerable-fortigate.example.com/admin";
malicious_input = "%x.%x.%x.%x.%n"
payload = {"command": malicious_input}

# Hypothetical POST endpoint
r = requests.post(url, data=payload, verify=False)
print(r.text)

*(Note: The actual endpoint and field names will depend on the interface and have not been published for safety.)*

Real-world Impact

- Remote Code Execution: An attacker can run any command the web/CLI/API process can, potentially gaining device control.
- Information Leak: Attackers can read stack and memory data, which could include passwords or secrets.
- Device Takeover: Attackers could modify configs, add backdoors, or pivot further into the network.

Mitigation

Fortinet has released patched versions for all affected products. Upgrade immediately!

Official References

- Fortinet Advisory (FG-IR-23-084)
- NVD CVE-2023-29181 Listing
- MITRE CVE Record

Conclusion

CVE-2023-29181 is a dangerous format string vulnerability in several major Fortinet products. It allows attackers to leak sensitive memory or even run arbitrary commands just by sending crafted inputs. Given how widely these products are deployed, patching should be a top priority.

If you use any affected Fortinet products, update now and take emergency steps to lock down management interfaces in the meantime.


> Stay safe: Always keep security appliances up to date, and look for unusual input handling paths as part of your review of network gear.


*This article is for educational purposes only. Do not attempt exploitation without authorization.*

Timeline

Published on: 02/22/2024 10:15:08 UTC