Technical Analysis of Unpacking CVE-20

Unpacking CVE-20

This analysis focuses on a critical vulnerability, CVE-2023-20198, impacting Cisco IOS XE software. This flaw allowed unauthenticated attackers to create new, privileged user accounts on affected devices. Attackers gained full control over the compromised systems. This severe issue prompted widespread concern across the cybersecurity community.

What the Vulnerability Is and Its Impact

CVE-2023-20198 is a privilege escalation vulnerability in the web UI of Cisco IOS XE software. An unauthenticated attacker could exploit this flaw. They could create an account with privilege level 15 access. This grants full administrative control over the device.

The vulnerability's CVSS v3.1 score is 10.0. This rates it as critical. Affected products include Cisco devices running IOS XE software with the web UI feature enabled. Specific versions were not initially disclosed, but Cisco advised checking for the presence of the implant. The vulnerability allowed for persistent access.

The impact was immediate and severe. Compromised devices could be fully controlled by attackers. This included intercepting traffic, altering configurations, or launching further attacks.

Technical Root Cause Analysis

The root cause of CVE-2023-20198 involved an unauthorized write function in the web UI. Attackers exploited this function. They injected a local user account directly into the system. This bypasses standard authentication mechanisms. The flaw did not stem from a traditional buffer overflow or SQL injection. Instead, it was an architectural weakness allowing unauthorized configuration changes.

Specifically, threat actors deployed a Lua-based implant. This implant was a malicious script. It was written to the file system of the Cisco IOS XE device. The implant then allowed the creation of new local users. These users possessed privilege level 15. This is the highest administrative privilege. The implant leveraged a previously unknown method to write files. This method circumvented standard security controls for configuration changes.

The implant itself was a two-stage payload. The first stage created a new user with administrative privileges. The second stage was a backdoor. This backdoor allowed arbitrary command execution. It masqueraded as a legitimate system process. This sophisticated approach made detection challenging. It also ensured persistence across reboots.

Exploitation Mechanics

Exploitation began with an unauthenticated HTTP request to the web UI. This request targeted a specific URL path. This path was vulnerable to the unauthorized write operation. The request's body contained the malicious Lua script.

The initial payload created a local user account. This account had a username and password chosen by the attacker. It was assigned privilege level 15. For example, an attacker might create a user named "cisco" with a strong password. This user then had full administrative access.

# Example (conceptual, not actual exploit code)
# This represents the type of action, not a runnable command.
# Attackers sent a crafted HTTP POST request to a specific endpoint.
# The payload contained Lua code to create a new user.
POST /webui/path/to/vulnerable/endpoint HTTP/1.1
Host: [TARGET_IP]
Content-Type: application/x-www-form-urlencoded
Content-Length: [LENGTH]

lua_script_payload=function() local f=io.open("/flash/webui_wsma_auth.lua","w"); f:write("username cisco privilege 15 secret 0 YourPassword\n"); f:close(); end; os.execute("source /flash/webui_wsma_auth.lua")

After creating the privileged account, attackers deployed a second-stage implant. This implant was a custom backdoor. It resided in a specific directory on the device's file system. This backdoor enabled command injection. It allowed remote execution of arbitrary commands. This provided a persistent foothold. The backdoor could be invoked via specific HTTP requests. These requests would trigger the execution of system commands.

The exploitation did not require complex bypasses. It exploited an inherent design flaw. The flaw allowed direct manipulation of system configuration files. This was possible without proper authorization checks.

Detection: How to Check if You're Affected

Detection of CVE-2023-20198 requires immediate action. Organizations must check for specific indicators of compromise (IoCs). Cisco released a script to detect the implant.

First, inspect your device logs for suspicious account creations. Look for new local users with high privilege levels. These accounts might appear without administrative approval. Check authentication logs for logins from unknown IP addresses.

Second, examine the file system for the presence of the malicious implant. The implant often resides in specific directories. Cisco identified /usr/binos/conf/nginx-conf/cisco_service.conf as a common location. It also used /flash/webui_wsma_auth.lua. Look for unexpected files with recent modification dates.

Cisco provided a script to check for the implant. Run this script on your IOS XE devices. It searches for known malicious files and configurations.

# Example command to check for the implant (conceptual, based on Cisco's advice)
# This command should be run in the Cisco IOS XE CLI.
show running-config | include "webui_wsma_auth"
show file system | include "cisco_service.conf"
more bootflash:webui_wsma_auth.lua

Network traffic analysis can also reveal compromise. Look for unusual outbound connections from your Cisco devices. These might indicate command-and-control (C2) communications. Monitor for unexpected HTTP requests to the device's web UI. Specifically, check for requests targeting the paths involved in the initial exploit.

Secably offers tools to assist with detection. Our free website vulnerability scanner can identify exposed web UIs. It checks for common misconfigurations that might lead to such vulnerabilities. Use our free port scanner to identify open ports. This helps confirm if the web UI is exposed to the internet. Secably's paid monitoring plans, starting at $19/month, can continuously scan your external attack surface. They alert you to newly exposed services or potential vulnerabilities like those related to Unpacking CVE-20. You can also use Zondex for internet-wide scanning. It can identify exposed Cisco IOS XE web UIs. This helps in proactive discovery of vulnerable assets.

Remediation Steps

Immediate remediation is critical for CVE-2023-20198. Follow these steps to secure your Cisco IOS XE devices.

1. Disable the Web UI: If not essential, disable the HTTP/HTTPS server for the web UI. This removes the attack vector. Use the no ip http server and no ip http secure-server commands in global configuration mode.
2. Upgrade IOS XE Software: Apply the official Cisco patches. Cisco released fixed software versions. Refer to the Cisco Security Advisory for specific version recommendations. Upgrade to a patched release as soon as possible.
3. Remove Malicious Users and Files: If compromised, remove any unauthorized user accounts. Use the no username [username] command. Delete any malicious files, such as webui_wsma_auth.lua or cisco_service.conf.
4. Change Credentials: Rotate all administrative passwords on the device. Assume all credentials might be compromised. Implement strong, unique passwords.
5. Monitor and Audit: Continuously monitor your device logs for suspicious activity. Implement robust auditing. Review configurations regularly.
6. Network Segmentation: Isolate management interfaces from public networks. Use strict access control lists (ACLs). Limit access to the web UI from trusted IP addresses only.

For further reading on similar threats, review our analysis on Unpacking CVE-2023-20198 — Cisco IOS XE Web UI Privilege Escalation. This details the specifics of the actual CVE. You can also check our post on Exploiting Ivanti Connect Secure Flaws What for insights into similar critical vulnerabilities.

Timeline of Disclosure

The timeline for CVE-2023-20198 unfolded rapidly. Threat actors actively exploited this vulnerability before public disclosure.

• October 12, 2023: Cisco Talos identified suspicious activity. They observed an unknown actor exploiting a zero-day vulnerability. This activity targeted Cisco IOS XE devices.
• October 16, 2023: Cisco published a security advisory (cisco-sa-iosxe-webui-privesc-j22MTd8GM). This advisory warned customers about active exploitation. It provided initial detection guidance. Cisco did not have a patch available at this time.
• October 18, 2023: CISA issued an alert (AA23-291A). This alert urged federal agencies to address the vulnerability. It highlighted the critical nature of the threat.
• October 22, 2023: Cisco provided further updates. They included additional indicators of compromise. They also detailed methods for implant detection.
• October 25, 2023: Cisco released software updates. These updates addressed the vulnerability. They provided patches for affected Cisco IOS XE versions.

The rapid exploitation and subsequent disclosure highlight the importance of proactive security measures. Organizations faced immediate threats. Quick response was essential to mitigate damage. The incident underscores the need for continuous monitoring and prompt patching.