CVE-2024-56326: Palletsprojects Jinja RCE Vulnerability

CVE-2024-56326 Overview

CVE-2024-56326 is a sandbox bypass vulnerability in Jinja, the popular extensible templating engine for Python. Prior to version 3.1.5, an oversight in how the Jinja sandboxed environment detects calls to str.format allows an attacker who controls the content of a template to execute arbitrary Python code. This vulnerability enables attackers to escape the sandbox protections that are designed to prevent untrusted template content from executing malicious operations.

Critical Impact

Attackers controlling template content can execute arbitrary Python code by bypassing Jinja's sandbox protections through indirect calls to str.format, potentially leading to complete system compromise.

Affected Products

• Palletsprojects Jinja versions prior to 3.1.5
• Applications using Jinja that execute untrusted templates
• Systems with custom Jinja filters that may call format methods on strings

Discovery Timeline

• 2024-12-23 - CVE CVE-2024-56326 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-56326

Vulnerability Analysis

This vulnerability represents a Protection Mechanism Failure (CWE-693) in Jinja's sandbox implementation. The sandbox is designed to provide a secure environment for executing untrusted templates by restricting access to potentially dangerous Python functionality. While Jinja's sandbox was implemented to catch direct calls to str.format and ensure they don't escape the sandbox, it failed to account for indirect invocation patterns.

The vulnerability allows attackers to store a reference to a malicious string's format method and then pass that reference to a filter that calls it. While no such filters are built into Jinja itself, custom filters in applications could inadvertently enable this attack vector. This indirect call bypasses the sandbox's security checks because the detection mechanism was only designed to intercept direct str.format invocations.

Root Cause

The root cause lies in the incomplete implementation of the sandbox's format string detection mechanism. The sandbox was designed to intercept and validate calls to str.format to prevent format string attacks that could lead to information disclosure or code execution. However, the implementation only checked for direct method calls and did not account for scenarios where a reference to the method is stored and called indirectly through another function or filter.

Attack Vector

The attack requires local access and the ability to control template content. The attacker exploits the vulnerability by:

1. Crafting a malicious template that stores a reference to a string's format method
2. Passing this stored reference to a custom filter that calls its argument
3. The filter invokes the format method, bypassing sandbox detection
4. The attacker can then access Python internals and execute arbitrary code

The exploitation depends on the presence of custom filters in the application that call their arguments as functions. Without such filters, built-in Jinja functionality alone cannot trigger this vulnerability.

# Security patch in src/jinja2/sandbox.py - Merge commit from fork
 from _string import formatter_field_name_split  # type: ignore
 from collections import abc
 from collections import deque
+from functools import update_wrapper
 from string import Formatter
 
 from markupsafe import EscapeFormatter

Source: GitHub Commit Update

Detection Methods for CVE-2024-56326

Indicators of Compromise

• Unusual template content containing references to string format methods being stored in variables
• Template execution logs showing attempts to access Python internals through format string manipulation
• Custom filter invocations with callable arguments that weren't expected

Detection Strategies

• Audit custom Jinja filters for patterns that call their arguments as functions
• Implement template content scanning for suspicious format method reference patterns
• Monitor application logs for sandbox-related errors or unusual template execution behavior
• Review template sources for untrusted input being used in template content

Monitoring Recommendations

• Enable verbose logging for Jinja template rendering operations
• Implement runtime monitoring for unexpected Python code execution within template contexts
• Set up alerts for template rendering failures that may indicate exploitation attempts
• Track and audit all custom filter implementations in your Jinja environment

How to Mitigate CVE-2024-56326

Immediate Actions Required

• Upgrade Jinja to version 3.1.5 or later immediately
• Audit all custom Jinja filters for patterns that could enable indirect method calls
• Review template sources to ensure untrusted content is not rendered as templates
• Implement strict input validation for any user-controlled template content

Patch Information

The vulnerability is fixed in Jinja version 3.1.5. The patch modifies the sandbox implementation to handle indirect calls to str.format by wrapping and monitoring method references. The fix ensures that even when a format method reference is passed through filters or other intermediaries, the sandbox properly intercepts and validates the call.

Patch details are available in the GitHub Security Advisory GHSA-q2x7-8rv6-6q7h and the security commit.

Workarounds

• Restrict template content to trusted sources only until patching is complete
• Remove or disable custom filters that call their arguments as functions
• Implement additional input sanitization layers before template content reaches Jinja
• Consider using Jinja's autoescape features and additional security extensions as defense in depth

# Configuration example
# Upgrade Jinja to the patched version
pip install --upgrade Jinja2>=3.1.5

# Verify the installed version
pip show Jinja2 | grep Version

# For requirements.txt, update the dependency
echo "Jinja2>=3.1.5" >> requirements.txt