Cisco NX-OS: The Call is Coming From Inside the Logs

Every developer has been there. You're debugging a complex authentication flow, nothing is working, and out of frustration, you flip the logging switch to 'ALL'. You need to see exactly what's hitting the wire. It works, you fix the bug, and you go home. But did you remember to turn the logging back down? Did you sanitize the input variables before writing them to disk?

CVE-2025-20290 is the architectural equivalent of leaving that debug switch taped to the 'ON' position. Cisco's NX-OS, the operating system powering the backbone of countless data centers via Nexus switches, decided that secrets were meant to be shared—specifically with the local filesystem. This isn't a complex heap overflow or a race condition in the kernel. It is a failure of discipline.

The vulnerability resides in the logging mechanism itself. When specific system operations occur—likely during provisioning, authentication handshakes, or configuration merges—the system writes sensitive data into log files. These aren't encrypted vaults; they are standard text files sitting in directories like /var/sysmgr/log/. For a hacker, this is the best kind of vulnerability: no ROP chains, no heap grooming, just cat and grep.

The root cause here is mapped to CWE-532: Insertion of Sensitive Information into Log File. It sounds mundane, but in the context of a network appliance, it is catastrophic. Network devices are fortresses; the outer walls are thick, but the interior assumes a level of trust. The flaw specifically affects how NX-OS handles data sanitation before writing to the syslog or internal component logs.

In a correct implementation, sensitive parameters like passwords, API tokens, or SNMP community strings should be masked (e.g., ******) or redacted entirely before being passed to the logging daemon. In the affected versions of NX-OS (spanning 7.x to 10.x) and UCS software, this sanitation step was missed in certain code paths.

What makes this dangerous is the persistence. Logs aren't ephemeral; they persist on disk until rotated. A credential logged three days ago is just as valid as one logged three seconds ago. If an attacker gains a foothold today, they can scroll back through time to find the keys to the kingdom.

Because Cisco NX-OS is proprietary, we don't have the public git commit to point and laugh at. However, we can reconstruct the logic failure with high fidelity based on the behavior. This is a logic bug, not a memory safety bug, so the code looks deceptively normal.

The Vulnerable Logic (Conceptual):

// A hypothetical logging function in the auth manager
void handle_auth_request(char *username, char *password) {
    // MISTAKE: Logging raw inputs for "debugging"
    syslog(LOG_INFO, "Processing auth for user: %s with pass: %s", username, password);
    
    if (authenticate(username, password)) {
        grant_access();
    } else {
        log_failure(username);
    }
}

In the snippet above, the developer likely intended to remove the %s for the password before shipping, or assumed that the log level LOG_INFO wouldn't be captured in production.

The Patched Logic:

// The fixed implementation sanitizes data first
void handle_auth_request(char *username, char *password) {
    // FIX: Never log the raw secret. 
    syslog(LOG_INFO, "Processing auth for user: %s", username);
    
    if (authenticate(username, password)) {
        grant_access();
    } else {
        log_failure(username);
    }
}

The patch likely involves searching the codebase for every instance of logging functions and ensuring that variables holding CREDENTIAL, TOKEN, or PASSWORD types are passed through a mask_secret() function or omitted entirely. In the context of the Cisco fix, they likely also had to implement a log scrubber to clean up existing logs on update, ensuring that applying the patch creates a clean slate.

Exploiting CVE-2025-20290 is delightfully low-tech. The barrier to entry isn't technical skill; it's access. You need to be authenticated, but you only need low privileges. Let's look at the attack path for a Cisco Nexus 9000.

Step 1: Gain Shell Access

First, we need to get to the underlying Linux shell. On NX-OS, this is often done via the guestshell feature or, if we have a basic operator account, dropping into bash.

switch> enable
switch# guestshell
[guestshell@guestshell ~]$ whoami
guest_user

Step 2: The Treasure Hunt

Once we are in the shell, we are effectively on a Linux box. We want to look where the system manager (sysmgr) keeps its notes. The advisory points to system logs. A simple grep command can reveal the jewels.

# Searching recursively for common credential patterns
grep -rE "password|secret|token|key" /var/sysmgr/log/ 
 
# Output might look like:
# /var/sysmgr/log/auth_debug.log: 2025-08-20 10:00:01: User 'admin' password set to 'SuperSecretCisco123!'

Step 3: The UCS Variation

On Cisco UCS Fabric Interconnects, you might not get a direct shell easily. However, the vulnerability disclosure notes that the logs are included in Tech Support Files.

1. Log in with a low-priv user allowed to generate reports.
2. Trigger a "Show Tech-Support" generation.
3. Download the tarball to your local machine.
4. Extract and search: tar -xvf tech-support.tar && grep -r "password" .

This is a "delayed read" exploit. You don't need to be on the box to parse the logs if the box kindly packages them up and lets you download them.

Why is a CVSS 5.5 dangerous? Because metrics lie. The AV:L (Local Attack Vector) pulls the score down significantly, implying you need physical access or a prior compromise to use this. While true, it ignores the reality of modern network administration.

In many environments, "Level 1" support engineers or automated monitoring scripts have low-level access to these switches to check interface statuses. If any of those accounts are compromised—or if a disgruntled employee decides to look around—this vulnerability turns that limited access into full root or admin control.

Once an attacker recovers the admin credentials from the log:

1. They can pivot to other devices (admin passwords are notoriously reused across fleet devices).
2. They can redirect traffic, mirroring sensitive flows to a collection point.
3. They can install persistent backdoors that survive the inevitable firmware update.

It is a classic Privilege Escalation. You start in the lobby, read the sticky note with the safe code, and walk into the vault.

Mitigation here is binary: you patch, or you remain vulnerable. There is no configuration workaround because you cannot easily instruct a closed-source binary to stop logging specific lines without breaking the binary itself.

Remediation Steps:

1. Upgrade NX-OS: Move to the fixed releases specified in Cisco Advisory cisco-sa-nxos-infodis-TEcTYSFG. Typically, this means jumping to the latest maintenance release of the 9.x or 10.x train.
2. Upgrade UCS Manager: Ensure Fabric Interconnects are running versions > 4.2(3) or 4.3(2) as appropriate.
3. Log Rotation/Deletion: This is critical. Applying the patch stops future leaks, but it might not delete past logs. After patching, force a log rotation or manually clear the log directories (clear logging logfile or similar CLI commands) to ensure the historical evidence is destroyed.
4. Credential Rotation: Assume your admin passwords have been read. If you find your device was running a vulnerable version, rotate all administrative credentials immediately after patching. Do not skip this step.