CVE-2021-33357: RaspAP RCE Vulnerability via GET Parameter

CVE-2021-33357 Overview

CVE-2021-33357 is a critical command injection vulnerability affecting RaspAP versions 2.6 through 2.6.5. The vulnerability exists in the iface GET parameter within /ajax/networking/get_netcfg.php, where insufficient input validation allows unauthenticated attackers to inject and execute arbitrary operating system commands by including special characters such as semicolons (;) in the parameter value.

RaspAP is a popular open-source wireless router software project that transforms Raspberry Pi devices into wireless access points. This vulnerability is particularly concerning as it requires no authentication, allowing any network-adjacent or remote attacker to achieve full system compromise on affected devices.

Critical Impact

Unauthenticated remote attackers can execute arbitrary OS commands on the underlying system, potentially leading to complete device takeover, data exfiltration, lateral movement within networks, and persistent backdoor installation.

Affected Products

• RaspAP version 2.6
• RaspAP version 2.6.1 through 2.6.5
• Devices running affected RaspAP versions (primarily Raspberry Pi-based access points)

Discovery Timeline

• 2021-06-09 - CVE-2021-33357 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-33357

Vulnerability Analysis

This command injection vulnerability (CWE-78) occurs due to improper neutralization of special elements used in an OS command. The vulnerable endpoint /ajax/networking/get_netcfg.php accepts user-supplied input through the iface GET parameter and passes it directly to system command execution without adequate sanitization or validation.

When a user supplies a network interface name, the application constructs a system command using this input. By injecting shell metacharacters like semicolons (;), pipes (|), or command substitution operators, an attacker can break out of the intended command context and execute arbitrary commands with the privileges of the web server process.

The vulnerability is particularly severe because it requires no authentication, meaning any attacker who can reach the web interface can immediately exploit this flaw. Given that RaspAP runs on Raspberry Pi devices often deployed as network access points, successful exploitation grants attackers a foothold in the target network.

Root Cause

The root cause is insufficient input validation and sanitization in the get_netcfg.php script. The iface parameter is used directly in system command construction without proper escaping or allowlist validation. The application fails to verify that the provided interface name matches a legitimate network interface format, nor does it escape shell metacharacters before command execution.

Proper mitigation would involve implementing strict allowlist validation against known interface names or using parameterized command execution methods that prevent shell interpretation of user input.

Attack Vector

The attack vector is network-based and does not require any user interaction or authentication. An attacker can exploit this vulnerability by sending a crafted HTTP GET request to the vulnerable endpoint with a malicious iface parameter value.

The attack flow involves:

1. Identifying a target running vulnerable RaspAP versions (2.6 to 2.6.5)
2. Crafting a request to /ajax/networking/get_netcfg.php with the iface parameter containing command injection payload
3. Using shell metacharacters (;, |, $(...), backticks) to append arbitrary commands
4. Commands execute with the privileges of the web server process

For technical details on the vulnerable code and exploitation methodology, refer to the GitHub Gist Exploit Example and the RaspAP source code.

Detection Methods for CVE-2021-33357

Indicators of Compromise

• HTTP GET requests to /ajax/networking/get_netcfg.php containing shell metacharacters (;, |, &, backticks, $(...)) in the iface parameter
• Unexpected child processes spawned by the web server process (e.g., www-data or apache2)
• Anomalous outbound network connections from the RaspAP device
• Unauthorized file modifications or new files in web-accessible directories
• Unexpected cron jobs or persistence mechanisms on the device

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing shell metacharacters in the iface parameter
• Monitor web server access logs for requests to /ajax/networking/get_netcfg.php with suspicious parameter values
• Deploy endpoint detection and response (EDR) solutions to identify command injection patterns and anomalous process execution
• Use network intrusion detection systems (NIDS) to identify exploitation attempts based on known payload signatures

Monitoring Recommendations

• Enable verbose logging on the RaspAP web server and centralize logs for analysis
• Implement file integrity monitoring on critical system directories
• Monitor for process execution anomalies, particularly shell commands spawned by the web server
• Configure alerts for any outbound connections from the RaspAP device to unexpected destinations

How to Mitigate CVE-2021-33357

Immediate Actions Required

• Upgrade RaspAP to a version newer than 2.6.5 where this vulnerability has been addressed
• Restrict network access to the RaspAP web interface to trusted IP addresses only
• If upgrading is not immediately possible, disable or block access to /ajax/networking/get_netcfg.php
• Audit systems for indicators of prior compromise before applying patches

Patch Information

Users should upgrade to the latest version of RaspAP beyond version 2.6.5. The vulnerable code exists in the /ajax/networking/get_netcfg.php file. Review the RaspAP GitHub repository for the current implementation and ensure your installation is running a patched version.

Workarounds

• Implement network-level access controls to restrict access to the RaspAP web interface to authorized administrators only
• Deploy a reverse proxy with input validation rules to filter malicious requests before they reach the application
• Disable the vulnerable endpoint entirely if the network configuration functionality is not required
• Place the RaspAP device behind a VPN to limit exposure to trusted users

# Example: Restrict access to RaspAP web interface using iptables
# Allow only specific trusted IP addresses
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP