CVE-2025-27516 is a vulnerability in the Jinja templating engine, a widely used extensible template engine for Python applications. The issue stems from an oversight in the sandboxed environment's handling of the |attr filter, which allows an attacker who controls template content to execute arbitrary Python code. Normally, Jinja's sandbox prevents dangerous operations by restricting access to certain methods, including str.format, which could be used for code execution. However, the vulnerability allows bypassing these restrictions by using the |attr filter to obtain a reference to the unprotected string format method, circumventing the sandbox's protections. This leads to arbitrary code execution within the context of the application running the template. Exploitation requires that the attacker can supply or manipulate the template content, which is common in applications that render user-generated templates or allow template customization. The vulnerability affects all Jinja versions prior to 3.1.6, where the fix was implemented by preventing the |attr filter from bypassing the sandbox's attribute lookup restrictions. The CVSS 4.0 score is 5.4 (medium severity), reflecting the requirement for local access or limited privileges and user interaction, but with high impact on confidentiality, integrity, and availability if exploited. No known exploits have been reported in the wild, but the vulnerability poses a significant risk in environments where untrusted templates are processed. Organizations using Jinja in web applications, especially those that allow dynamic or user-supplied templates, should prioritize patching and review template handling practices.

For European organizations, this vulnerability can lead to arbitrary code execution on servers running vulnerable Jinja versions, potentially compromising confidentiality, integrity, and availability of critical systems. Attackers exploiting this flaw could execute malicious Python code, leading to data breaches, unauthorized access, or disruption of services. This is particularly impactful for industries relying on web applications that utilize Jinja for dynamic content rendering, such as finance, healthcare, and government services. The medium severity rating indicates that exploitation is not trivial and requires some level of access or user interaction, but the consequences of a successful attack are severe. Given the widespread use of Python and Jinja in European software development, especially in web frameworks like Flask, the vulnerability could affect a broad range of applications. Failure to patch could expose organizations to targeted attacks or supply chain risks if third-party software incorporates vulnerable Jinja versions. Additionally, regulatory compliance in Europe, such as GDPR, could be impacted if personal data is compromised due to exploitation of this vulnerability.

1. Upgrade all Jinja instances to version 3.1.6 or later immediately to apply the official fix. 2. Audit all applications using Jinja to identify where templates are sourced and ensure that untrusted or user-supplied templates are not processed without strict validation or sanitization. 3. Implement strict input validation and sanitization on any user input that could influence template content. 4. Employ runtime application self-protection (RASP) or web application firewalls (WAF) with rules to detect and block suspicious template injection attempts. 5. Review and restrict permissions of application environments to limit the impact of potential code execution, such as running applications with least privilege. 6. Monitor application logs for unusual template rendering activity or errors that could indicate exploitation attempts. 7. Educate developers about secure template usage and the risks of processing untrusted templates. 8. Where possible, isolate template rendering environments to contain potential exploitation. 9. For legacy systems that cannot be immediately updated, consider disabling or restricting the |attr filter usage if feasible. 10. Maintain an incident response plan to quickly address any signs of compromise related to this vulnerability.