In late 2022, Cisco announced a severe vulnerability affecting its Enterprise NFV Infrastructure Software (NFVIS) — a crucial backbone for virtualization in networking hardware. Labeled as CVE-2022-20929, this flaw can allow a local, unauthenticated attacker to upload an unsigned or altered software upgrade file. If exploited, it could give the attacker total control over the device. The core issue? Cisco didn’t properly check the cryptographic signature on upgrade packages.

Let’s break down what went wrong, how attackers might exploit it, and why it matters.

What is NFVIS and Why Does It Matter?

Cisco’s NFV Infrastructure Software lets customers run network functions (like routers and firewalls) as virtual machines. Imagine your internet traffic, banking data, or sensitive corporate communications running behind one of these Cisco boxes in a server room.

When there’s a new version or patch, administrators upload an upgrade file through a web interface or command line. Normally, these files are signed to verify that they’re official Cisco code and not something tampered with or malicious.

Understanding the Vulnerability

The vulnerability appears because of insufficient cryptographic signature verification. In simple terms: Cisco’s software should have checked to make sure every upgrade file was really signed by Cisco, but it didn’t do this properly.

Here’s what the Cisco advisory says:

> “A vulnerability in the upgrade signature verification of Cisco Enterprise NFV Infrastructure Software (NFVIS) could allow an unauthenticated, local attacker to provide an unauthentic upgrade file for upload. This vulnerability is due to insufficient cryptographic signature verification of upgrade files.”

How Does the Exploit Work?

Attackers with any kind of LOCAL access—whether through a compromised account or physical access—can prepare a malicious file. They then convince an administrator to upload and install this file as an update.

Here's how a typical attack would unfold

1. Create a fake upgrade file: The attacker crafts a file that looks like a Cisco upgrade package but contains malicious code (like a reverse shell or a backdoor).
2. Present it to the admin: The attacker uses social engineering—maybe claiming it's an urgent hotfix from Cisco.
3. Upload and install: Because Cisco’s NFVIS doesn’t verify the signature properly, it accepts and installs the malicious update.
4. Full device compromise: After installation, the attacker’s code runs with high-level privileges, potentially owning the entire device.

Anyone with local access can do this—no login required. In environments where admins rely on files downloaded or handed over by third parties, this is a big risk.

What Does the Vulnerable Code Look Like?

Below is a *simplified* version of what might happen in affected NFVIS code (for demonstration purposes):

def verify_upgrade_file(file_path):
    # Let's say the code tries to check a file signature
    try:
        signature = extract_signature(file_path)
        verified = cryptographic_check(signature, file_path)
        if verified:
            return True
        else:
            print("Warning: Signature not verified.")
            # In the real flaw, the process keeps going anyway!
            return True  # <-- Should be 'False'. Here is the bug!
    except Exception as e:
        # If signature extraction fails, just continue
        print(f"Signature exception: {e}")
        return True  # <-- Another failure point

In the vulnerable logic, even if verification fails, the code still returns True, green-lighting the file's installation.

How Would a Malicious Upgrade Package Look?

Cisco upgrade files are typically ZIP or TAR archives with scripts and binaries, plus a signature file. An attacker can create a package like this:

$ mkdir fake_upgrade
$ echo "Malicious script here" > fake_upgrade/install.sh
$ tar -czvf fake_upgrade.tar.gz fake_upgrade/

Then, using standard tools or scripts, they can “fake” a signature or even leave it blank. With the bug in play, the NFVIS software doesn't block the install.

Here's* a simulation (not a real exploit) where a local attacker uses CLI access

# Attacker with local shell puts a malicious file in the temp folder
cp /tmp/fake_upgrade.tar.gz /var/tmp/
# Calls the admin or sends an email: "This is the urgent patch"
# Admin uploads through the GUI, and the system doesn't check the signature properly

Afterward, their malicious script could create a root-level backdoor, open network ports, or exfiltrate configs.

Technical Impact

- Attacker gains full root/admin access to the NFVIS appliance.

Update to a fixed version

Download latest NFVIS from the official Cisco Software Download page.

References

- Cisco Security Advisory: CVE-2022-20929
- NIST NVD: CVE-2022-20929

Conclusion

While CVE-2022-20929 requires some local access, it’s still a major risk—especially in networks where admins might handle update files provided by others. It’s a strong lesson in why cryptographic checks on software are critical, and why we must never trust unsigned code.

If you run Cisco NFVIS, patch now and stay alert to the sources of your update files. Every crack in the armor can become a gaping hole if attackers are crafty enough.

Timeline

Published on: 03/10/2023 21:15:00 UTC
Last modified on: 03/16/2023 13:22:00 UTC