CVE-2021-26120: Smarty Template Engine RCE Vulnerability

CVE-2021-26120 Overview

CVE-2021-26120 is a critical code injection vulnerability affecting Smarty, a popular PHP template engine. The flaw exists in versions prior to 3.1.39 and allows attackers to inject arbitrary code via an unexpected function name after a {function name= substring within Smarty templates. This vulnerability enables remote attackers to execute arbitrary PHP code on systems running vulnerable Smarty installations without requiring authentication or user interaction.

Critical Impact

This code injection vulnerability allows unauthenticated remote attackers to execute arbitrary PHP code on affected systems, potentially leading to complete server compromise, data theft, or use of the server for further attacks.

Affected Products

• Smarty versions prior to 3.1.39
• Debian Linux 9.0
• Debian Linux 10.0
• Debian Linux 11.0

Discovery Timeline

• 2021-02-22 - CVE-2021-26120 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-26120

Vulnerability Analysis

This vulnerability is classified as CWE-94 (Improper Control of Generation of Code, or Code Injection). The flaw resides in how Smarty parses and processes the {function} tag within templates. When processing template directives, the Smarty compiler fails to properly validate function names that follow the {function name= syntax. An attacker can craft a malicious template containing unexpected characters or constructs after this substring, which the parser incorrectly interprets as legitimate code to be executed.

The vulnerability is particularly dangerous because Smarty templates are compiled into PHP code for execution. When malicious input bypasses the validation mechanisms, the injected code is incorporated directly into the compiled template and subsequently executed with the same privileges as the PHP process handling the request.

Root Cause

The root cause of CVE-2021-26120 lies in insufficient input validation within Smarty's template compiler. The {function} tag handler does not adequately sanitize or validate the function name parameter before incorporating it into the generated PHP code. This allows specially crafted function names to break out of the intended context and inject arbitrary PHP statements into the compiled template output.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a web application that uses a vulnerable version of Smarty
2. Finding an input vector that allows template content to be influenced (e.g., user-controlled template data, CMS functionality, or template injection points)
3. Injecting a malicious {function name= directive with crafted content designed to escape the function definition context
4. Triggering the template compilation process, which incorporates the malicious code into the generated PHP
5. The injected code executes when the compiled template is processed

The vulnerability mechanism exploits the template parsing logic where function name validation is insufficient. When a malformed {function name= directive is encountered, the parser fails to properly sanitize the subsequent content, allowing PHP code injection into the compiled template output. For technical details on the specific parsing flaw, see the Smarty CHANGELOG.

Detection Methods for CVE-2021-26120

Indicators of Compromise

• Unusual {function name= patterns in template files or web request logs containing special characters or PHP syntax
• Unexpected PHP code execution from web server processes
• Modified or newly created files in temporary directories used by Smarty for compiled templates
• Web server error logs showing parsing errors or unusual template compilation failures
• Outbound network connections from the web server to unknown destinations

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block requests containing suspicious {function name= patterns with injection attempts
• Monitor Smarty compiled template directories for unexpected file modifications or malicious PHP code
• Implement file integrity monitoring on template directories and Smarty cache locations
• Analyze web application logs for unusual template-related requests or error patterns
• Use static code analysis tools to scan templates for potentially malicious {function} tag usage

Monitoring Recommendations

• Enable verbose logging for PHP applications using Smarty to capture template compilation events
• Configure intrusion detection systems (IDS) to alert on patterns associated with PHP code injection
• Implement real-time monitoring of web server process behavior for signs of code execution exploitation
• Set up alerts for any changes to compiled template files outside of normal deployment windows

How to Mitigate CVE-2021-26120

Immediate Actions Required

• Upgrade Smarty to version 3.1.39 or later immediately on all affected systems
• Audit all templates for potentially malicious {function} tag usage
• Review and restrict user input that may influence template content
• Implement input validation and sanitization for any user-controllable data that could reach template processing
• Consider temporarily disabling dynamic template features if immediate patching is not possible

Patch Information

The vulnerability has been addressed in Smarty version 3.1.39. Organizations should upgrade to this version or later to remediate the vulnerability. The fix implements proper validation of function names within the {function} tag handler, preventing the injection of arbitrary code through malformed function names.

For detailed patch information, refer to the Smarty CHANGELOG. Distribution-specific patches are available for Debian systems as documented in DSA-5151 and Gentoo GLSA 202105-06.

Workarounds

• Implement strict input validation to reject any template content containing {function directives from untrusted sources
• Use a web application firewall (WAF) to filter requests containing suspicious Smarty template syntax
• Restrict file system permissions on template directories to prevent unauthorized template modifications
• Disable user-controllable template functionality if upgrading is not immediately feasible
• Consider sandboxing Smarty template compilation in isolated environments with limited privileges

# Verify installed Smarty version
composer show smarty/smarty | grep versions

# Upgrade Smarty to patched version
composer require smarty/smarty:^3.1.39

# For Debian systems, apply security updates
sudo apt-get update && sudo apt-get install --only-upgrade smarty3