CVE-2020-15227: Nette Application RCE Vulnerability

CVE-2020-15227 Overview

CVE-2020-15227 is a critical code injection vulnerability affecting multiple versions of the Nette PHP/Composer MVC Framework. The vulnerability exists in how the framework processes specially crafted URL parameters, allowing attackers to inject and execute arbitrary code on vulnerable servers. This flaw can potentially lead to full Remote Code Execution (RCE), giving attackers complete control over affected systems.

Critical Impact

Unauthenticated remote attackers can achieve full server compromise through code injection via maliciously crafted URL parameters, potentially leading to data theft, system takeover, and lateral movement within networks.

Affected Products

• Nette Application versions before 2.0.19
• Nette Application versions before 2.1.13, 2.2.10, 2.3.14, 2.4.16, 3.0.6
• Debian Linux 9.0

Discovery Timeline

• October 1, 2020 - CVE-2020-15227 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-15227

Vulnerability Analysis

This vulnerability is classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-94 (Improper Control of Generation of Code). The core issue stems from insufficient input validation when processing URL parameters within the Nette Framework's routing and request handling mechanisms.

When a user submits a request with specially crafted parameters, the framework fails to properly sanitize these inputs before incorporating them into code execution paths. This allows an attacker to break out of the intended parameter context and inject arbitrary PHP code that gets executed by the server.

The vulnerability is particularly dangerous because it requires no authentication and can be exploited remotely over the network. An attacker only needs to craft a malicious HTTP request with specially formed URL parameters to trigger the code injection.

Root Cause

The root cause of CVE-2020-15227 lies in improper input validation within the Nette Framework's URL parameter handling logic. The framework does not adequately sanitize or validate user-supplied input before processing it in a manner that allows code execution. This lack of proper neutralization of special elements enables attackers to inject code through seemingly normal URL parameters.

Attack Vector

The attack vector is network-based, requiring an attacker to send specially crafted HTTP requests to a vulnerable Nette-based application. The attacker constructs URL parameters containing malicious code sequences that, when processed by the vulnerable framework, result in arbitrary code execution.

The exploitation flow involves crafting URL parameters that exploit the parsing logic to inject PHP code. Since the framework fails to properly validate these parameters, the injected code is interpreted and executed by the PHP runtime, granting the attacker the ability to run arbitrary commands on the server.

For detailed technical information about the exploitation mechanism, refer to the GitHub Security Advisory.

Detection Methods for CVE-2020-15227

Indicators of Compromise

• Unusual or malformed URL parameters in web server access logs containing encoded PHP code fragments or shell commands
• Unexpected PHP processes spawning child processes, particularly command shells or network utilities
• Web application logs showing errors related to parameter parsing or code execution anomalies
• New or modified files in web application directories, especially PHP files with obfuscated content

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing code injection patterns in URL parameters
• Deploy application-level logging to capture all incoming request parameters for forensic analysis
• Use intrusion detection systems (IDS) with signatures for PHP code injection patterns
• Monitor for anomalous HTTP requests with unusual parameter lengths or encoding schemes

Monitoring Recommendations

• Enable verbose logging on web servers to capture full request details including all URL parameters
• Set up real-time alerting for requests matching code injection patterns targeting Nette applications
• Monitor system process trees for web server processes spawning unexpected child processes
• Implement file integrity monitoring on web application directories to detect unauthorized modifications

How to Mitigate CVE-2020-15227

Immediate Actions Required

• Identify all Nette Framework installations in your environment and determine their versions
• Upgrade immediately to patched versions: 2.0.19+, 2.1.13+, 2.2.10+, 2.3.14+, 2.4.16+, or 3.0.6+
• Implement WAF rules to block malicious URL parameter patterns as a temporary measure
• Review web server logs for signs of exploitation attempts

Patch Information

Nette has released security patches for all affected version branches. The fixed versions are:

• Version 2.0.19 and later for the 2.0.x branch
• Version 2.1.13 and later for the 2.1.x branch
• Version 2.2.10 and later for the 2.2.x branch
• Version 2.3.14 and later for the 2.3.x branch
• Version 2.4.16 and later for the 2.4.x branch
• Version 3.0.6 and later for the 3.0.x branch

Updates can be obtained through Composer via Packagist. Debian users should refer to the Debian LTS Announcement for distribution-specific patches.

Workarounds

• Deploy a Web Application Firewall with strict input validation rules to filter malicious URL parameters
• Implement application-level input validation to sanitize all URL parameters before processing
• Restrict network access to vulnerable applications using firewall rules until patching is complete
• Consider temporarily disabling affected applications if they are non-critical and cannot be immediately patched

# Composer update command to patch Nette Framework
composer update nette/application --with-dependencies

# Verify installed version after update
composer show nette/application | grep versions