CVE-2025-27516 is a medium-severity vulnerability affecting versions of the Jinja templating engine prior to 3.1.6. Jinja is widely used in Python web applications to generate dynamic content by rendering templates. The vulnerability arises from an oversight in the sandboxed environment's handling of the |attr filter, which allows an attacker who can control template content to execute arbitrary Python code. Specifically, while Jinja's sandbox prevents escaping via str.format calls, the |attr filter can be exploited to obtain a reference to a string's native format method, bypassing sandbox restrictions. This enables malicious template authors to execute arbitrary code within the application context. Exploitation requires the attacker to have the ability to supply or modify template content, which depends on the application's design and whether untrusted templates are rendered. The vulnerability was addressed in Jinja version 3.1.6 by modifying the |attr filter to respect the sandbox's attribute lookup rules, preventing this bypass. No known exploits are currently reported in the wild, but the flaw represents a significant risk for applications that render untrusted templates using vulnerable Jinja versions.

For European organizations, this vulnerability poses a risk primarily to web applications and services that utilize Jinja for template rendering and allow untrusted user input to influence template content. Successful exploitation could lead to arbitrary code execution, compromising confidentiality, integrity, and availability of affected systems. This can result in data breaches, unauthorized access, service disruption, and potential lateral movement within networks. Organizations in sectors such as finance, healthcare, government, and critical infrastructure that rely on Python-based web applications are particularly at risk. The medium CVSS score reflects the requirement for some privileges (low privileges) and user interaction, limiting the ease of exploitation but not eliminating the threat. Given the widespread use of Jinja in open-source and commercial Python applications, the vulnerability could have broad impact if unpatched. European entities must assess their software supply chain and internal applications for vulnerable Jinja versions to prevent exploitation.

1. Immediate upgrade to Jinja version 3.1.6 or later to apply the official patch that fixes the |attr filter sandbox bypass. 2. Audit all applications and services that use Jinja templating to identify those rendering templates from untrusted or user-controlled sources; restrict or sanitize template inputs to trusted content only. 3. Implement strict input validation and sanitization on any user-supplied data that might influence template rendering. 4. Employ runtime application self-protection (RASP) or web application firewalls (WAFs) with custom rules to detect and block suspicious template injection attempts. 5. Review and limit permissions of application processes running Jinja to minimize impact of potential code execution. 6. Monitor application logs for unusual template rendering errors or suspicious activity indicative of exploitation attempts. 7. Educate developers on secure template usage and sandboxing best practices to avoid similar vulnerabilities in the future.