CVE-2025-27516 - Arbitrary Code Execution Vulnerability by Bypassing Jinja's Sandboxed Environment

Jinja is a widely used extensible templating engine designed for Python applications, enabling developers to generate dynamic HTML, XML, or other markup surfaces. However, a vulnerability (CVE-2025-27516) has been discovered in Jinja versions prior to 3.1.6, which allows for arbitrary code execution by bypassing its sandboxed environment. This post will dive into the details of this vulnerability, its exploit method, and how to mitigate this issue.

Description

The vulnerability (CVE-2025-27516) lies in the interaction between Jinja's sandboxed environment and the |attr filter. A user that has control over the content of a template could exploit this vulnerability to execute arbitrary Python code.

Specifically, the issue occurs because Jinja's sandbox catches calls to str.format and ensures that they do not escape the sandbox. However, an attacker can bypass the sandbox by utilizing the |attr filter to obtain a reference to a string's plain format method, thereby allowing them to execute arbitrary code.

Affected Applications

This vulnerability affects users of applications that rely on Jinja to execute untrusted templates. The severity of the vulnerability and its potential exploitability depends on the specific application using Jinja.

Here's an example of how an attacker could exploit this vulnerability

{% set payload = "().__class__.__bases__[].__subclasses__()" %}
{{ ''|attr(payload)|attr('__getitem__')(59)|attr('__call__')()|attr('__init__')(None,'os.system')('YOUR_COMMAND_HERE') }}

In this example, the attacker uses the |attr filter to bypass the sandbox and obtain a reference to the format method of a string object. The attacker then uses it to execute their desired command (YOUR_COMMAND_HERE).

Mitigation

Upgrading Jinja to version 3.1.6 or later is recommended to mitigate this vulnerability, as this update ensures the |attr filter no longer bypasses the environment's attribute lookup.

To upgrade, use the following command:

pip install -U Jinja==3.1.6

Original References

- Jinja GitHub Repository
- Security Advisory for CVE-2025-27516
- Jinja 3.1.6 Release Notes

Conclusion

The discovery of CVE-2025-27516 highlights the potential risks associated with using extensible templating engines such as Jinja in applications that execute untrusted templates. It is essential for users of Jinja to upgrade their installations to version 3.1.6 or later to mitigate this arbitrary code execution vulnerability. By staying informed and proactive with software updates, developers can continue to build reliable and secure applications.

Timeline

Published on: 03/05/2025 21:15:20 UTC