CVE-2021-29454 is a vulnerability in the Smarty PHP template engine, specifically affecting versions prior to 3.1.42 and versions 4.0.0 up to but not including 4.0.2. Smarty is widely used to separate presentation logic (HTML/CSS) from application logic in PHP applications. The vulnerability arises from improper neutralization of special elements in output used by a downstream component, classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component - Injection). The issue specifically involves the 'math' function in Smarty templates, which allows template authors to perform mathematical operations. Before the patched versions, if user-supplied data was passed unchecked into the math function, an attacker could craft a malicious math string that would be interpreted as PHP code, leading to arbitrary code execution. This means that an attacker controlling input to the math function could execute arbitrary PHP commands on the server hosting the vulnerable Smarty engine. The vulnerability does not require authentication but does require that user input is passed into the math function in an unsafe manner. While no known exploits have been reported in the wild, the potential for remote code execution makes this a significant risk. The vulnerability was publicly disclosed on January 10, 2022, and patches are available in Smarty versions 3.1.42 and 4.0.2. Organizations using affected Smarty versions should upgrade promptly to mitigate this risk.

For European organizations, the impact of this vulnerability can be substantial, especially for those relying on PHP-based web applications that use Smarty as their templating engine. Successful exploitation could lead to remote code execution, allowing attackers to compromise the confidentiality, integrity, and availability of affected systems. This could result in data breaches, unauthorized access to sensitive information, defacement of websites, or full system compromise. Given the widespread use of PHP in web development across Europe, including in government, finance, healthcare, and e-commerce sectors, the vulnerability poses a risk to critical infrastructure and services. The ability to execute arbitrary PHP code remotely without authentication increases the attack surface and potential for automated exploitation if the vulnerability is not patched. Although no active exploits are currently known, the medium severity rating and the nature of the vulnerability warrant urgent attention to prevent future attacks.

Upgrade all Smarty installations to version 3.1.42 or 4.0.2 or later immediately to apply the official patch addressing this vulnerability. Review all template code to ensure that user input is never directly passed to the math function or any other functions that evaluate or execute code without proper sanitization and validation. Implement strict input validation and sanitization on all user-supplied data before it is processed by the template engine to prevent injection of malicious code. Conduct a thorough code audit of PHP applications using Smarty to identify and remediate unsafe usage patterns of the math function or other dynamic code execution features. Deploy web application firewalls (WAFs) with rules designed to detect and block suspicious payloads targeting template injection or PHP code execution attempts. Monitor application logs for unusual activity or errors related to template processing that could indicate attempted exploitation. Ensure that PHP error reporting is configured securely to avoid leaking sensitive information that could aid attackers in crafting exploits. Establish a patch management process that includes timely updates of third-party components such as Smarty to reduce exposure to known vulnerabilities.