CVE-2024-56326

What it actually does

This vulnerability is a sandbox escape in Jinja when untrusted template content is rendered with SandboxedEnvironment. The sandbox normally intercepts direct calls to str.format to prevent access to unsafe Python internals, but affected versions can miss indirect calls where a template first obtains a reference to a string object's format method and then causes application-provided code, such as a custom filter, to invoke that method. If an application allows attacker-controlled templates and includes a filter or extension that calls attacker-supplied callables, the attacker may break out of the intended template sandbox and execute arbitrary Python code with the privileges of the hosting application process.

Why it matters to the business

The practical risk is concentrated in applications that let users create, upload, edit, or otherwise control Jinja templates and rely on Jinja's sandbox as a security boundary. Successful exploitation can lead to server-side code execution, data theft, credential exposure, modification of generated content, service disruption, and lateral movement from the compromised application runtime. The vulnerability is not listed in CISA KEV, is not known to be used in ransomware campaigns, and has a low EPSS score, which reduces evidence of broad exploitation; however, the impact is high for exposed template-rendering features because exploitation can become full application compromise.

How an attack unfolds

An attacker first identifies an application feature that renders user-controlled Jinja templates inside the sandboxed environment, such as a report builder, notification template editor, CMS theme/template feature, workflow engine, or document-generation system. The attacker then submits a crafted template that abuses the sandbox's incomplete handling of indirect str.format calls. Instead of directly calling str.format, which the sandbox attempts to inspect, the template stores or passes a bound format method and relies on an application-defined filter, extension, or helper that invokes that callable. If such a helper exists, the call can occur outside the expected sandbox checks. From there, the attacker attempts to reach unsafe Python objects and execute commands or access sensitive data in the context of the application process. The attack usually requires experimentation against the application's custom filters and available template context, so exploitability is highly application-specific rather than universal across all Jinja deployments.

How to fix it

Upgrade Jinja to a version containing the fix for commit 877f6e51be8e1765b06d911cfaa9033775f051d1, or install a later official release that includes that commit. If package-managed releases are available, prefer the latest supported Jinja release rather than pinning directly to a Git commit. In addition, treat Jinja sandboxing as defense-in-depth rather than a complete isolation boundary: avoid rendering untrusted templates where possible; remove or tightly restrict custom filters/extensions that invoke attacker-controlled callables; deny access to dangerous objects in the template context; render untrusted templates in a separate, low-privilege worker or container; and enforce OS-level controls such as read-only filesystems, no shell access, restricted network egress, and minimal secrets in the rendering process.

How to confirm the fix worked

Confirm the installed Jinja package or source checkout includes commit 877f6e51be8e1765b06d911cfaa9033775f051d1 or a later fixed release. Inventory applications that import jinja2.sandbox.SandboxedEnvironment or expose template-editing features to non-administrative users. Review custom filters, tests, globals, and extensions for behavior that calls a callable, method, or function supplied by template content. In a non-production environment, run a safe regression test that attempts the previously vulnerable indirect str.format invocation pattern and verify that the sandbox blocks it or raises a security exception. Also validate that template-rendering workers run with least privilege and cannot access production secrets, shell utilities, or unnecessary network destinations.

Typical attack path

1. Discover an application feature that accepts or stores user-controlled Jinja template content.
2. Confirm that the application renders the template using Jinja SandboxedEnvironment rather than treating it as plain text.
3. Identify whether custom filters, tests, extensions, or helpers invoke callables or methods supplied from the template context.
4. Submit a crafted template that obtains an indirect reference to a string format method and routes it through such an invoking filter or helper.
5. Use the sandbox escape to access unsafe Python internals, sensitive application objects, or operating-system interfaces available to the process.
6. Execute code, read secrets, exfiltrate data, alter generated content, or establish persistence depending on the application permissions.

How to detect it

Historical exploitation detection is application-specific because the vulnerable behavior depends on whether untrusted templates and callable-invoking custom filters are present. Review audit logs for template creation, modification, preview, and rendering events, especially from unusual users, IPs, user agents, or at unusual times. Inspect stored templates and request bodies for server-side template injection patterns, indirect format-method usage, Python introspection strings, attempts to access built-ins, imports, subprocess execution, or environment variables. Review application logs for Jinja sandbox errors, TemplateRuntimeError, SecurityError, UndefinedError, or unusual exceptions occurring during template preview/render actions. On hosts or containers that render templates, examine process telemetry for unexpected child processes spawned by the Python application, such as shells, curl/wget, Python one-liners, or command interpreters. Review filesystem and secret-access logs for unexpected reads by the template-rendering process, and network telemetry for outbound connections from services that normally only render templates. If available, correlate template edit events with subsequent process execution, credential access, or outbound traffic.

• .format
• |attr(
• __globals__
• __builtins__
• __import__
• __mro__
• __subclasses__
• __class__
• __base__
• subprocess
• os.popen
• popen(
• eval(
• exec(
• open(
• /bin/sh
• /bin/bash
• cmd.exe
• powershell
• SecurityError
• TemplateRuntimeError
• SandboxedEnvironment
• jinja2.exceptions.SecurityError
• jinja2.exceptions.TemplateRuntimeError

What your security team should do

• Prioritize patching applications that render user-controlled Jinja templates, especially external-facing template builders, CMS/theme systems, report generators, email-template editors, and workflow automation platforms.
• Upgrade Jinja to a fixed build containing commit 877f6e51be8e1765b06d911cfaa9033775f051d1 or a later supported release, and rebuild/redeploy all dependent services.
• Search codebases for use of jinja2.sandbox.SandboxedEnvironment and for custom Jinja filters or extensions that invoke callables supplied from templates.
• Temporarily disable untrusted template editing or restrict it to trusted administrators until affected deployments are patched and reviewed.
• Harden template-rendering execution by isolating it in low-privilege containers or workers with restricted filesystem, environment variables, credentials, and network egress.
• Review logs and stored templates for Jinja sandbox-escape and server-side template injection patterns, especially around template creation or modification events.
• Add application-level monitoring for suspicious template syntax, sandbox security exceptions, unexpected child processes from Python workers, and outbound network connections from template-rendering components.