CVE-2024-56326 is a vulnerability in the Jinja templating engine, specifically versions before 3.1.5, caused by an oversight in the sandbox environment's detection of calls to Python's str.format method. Jinja's sandbox is designed to prevent execution of arbitrary code by restricting certain operations within templates. While direct calls to str.format are blocked, this vulnerability allows an attacker to bypass these restrictions by storing a reference to a malicious string's format method and passing it to a custom filter that invokes it. Since Jinja does not include such filters by default, exploitation depends on the presence of custom filters in the application that call str.format on user-controlled data. If an attacker controls the template content, they can leverage this flaw to execute arbitrary Python code, potentially compromising the host system. The vulnerability is tracked under CWE-1336 (Improper Neutralization of Special Elements Used in a Template Engine) and CWE-693 (Protection Mechanism Failure). The issue was resolved in Jinja version 3.1.5 by extending sandbox protections to cover indirect calls to str.format. The CVSS 4.0 score is 5.4 (medium), reflecting the requirement for local or limited access, partial privileges, and user interaction, but with high impact on confidentiality, integrity, and availability if exploited. No known exploits have been reported in the wild as of the publication date.

For European organizations, the impact of this vulnerability depends largely on their use of Jinja in web applications or services that process templates from untrusted sources. If exploited, attackers could execute arbitrary Python code, leading to full system compromise, data theft, or service disruption. This is particularly critical for organizations running internal or customer-facing applications that allow template customization or user-generated templates. The vulnerability could affect sectors such as finance, healthcare, government, and technology, where Python-based web frameworks (e.g., Flask, which uses Jinja) are common. Exploitation could result in breaches of sensitive personal data protected under GDPR, leading to regulatory penalties and reputational damage. Additionally, the ability to execute code could facilitate lateral movement within networks or deployment of ransomware. However, the requirement for control over template content and the presence of custom filters reduces the attack surface somewhat. Organizations using default Jinja configurations without custom filters are less at risk.

1. Upgrade all Jinja installations to version 3.1.5 or later immediately to apply the fix that extends sandbox protections. 2. Audit all applications using Jinja for custom filters, especially those that may invoke str.format or other string formatting methods on user-controlled input. 3. Restrict template editing or uploading capabilities to trusted users only, minimizing the risk of untrusted template injection. 4. Implement strict input validation and sanitization on any user inputs that influence template content. 5. Employ runtime application self-protection (RASP) or web application firewalls (WAFs) with rules targeting suspicious template-related activities. 6. Monitor application logs for unusual template rendering errors or suspicious filter invocations. 7. Educate developers on secure template usage and sandboxing principles to avoid introducing similar vulnerabilities. 8. Consider isolating template rendering environments using containerization or sandboxing at the OS level to limit potential damage from exploitation.