CVE-2025-27516: Palletsprojects Jinja RCE Vulnerability

CVE-2025-27516 Overview

CVE-2025-27516 is a sandbox escape vulnerability in Jinja, an extensible templating engine widely used in Python web applications. Prior to version 3.1.6, an oversight in how the Jinja sandboxed environment interacts with the |attr filter allows an attacker that controls the content of a template to execute arbitrary Python code.

The vulnerability stems from a bypass mechanism where attackers can use the |attr filter to obtain a reference to a string's plain format method, circumventing the sandbox protections that normally catch calls to str.format. This vulnerability impacts users of applications that execute untrusted templates.

Critical Impact

Attackers who control template content can escape Jinja's sandbox and execute arbitrary Python code on the underlying system, potentially leading to complete system compromise.

Affected Products

• Palletsprojects Jinja (versions prior to 3.1.6)
• Debian Linux 11.0

Discovery Timeline

• 2025-03-05 - CVE-2025-27516 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-27516

Vulnerability Analysis

The vulnerability exists in the interaction between Jinja's sandboxed environment and the |attr filter. Jinja's sandbox is designed to prevent untrusted template code from accessing dangerous Python functionality. Part of this protection includes catching calls to str.format to ensure they don't provide an escape vector.

However, the |attr filter was not properly integrated with the environment's attribute lookup mechanism. This oversight allows an attacker to use the |attr filter to retrieve a reference to the plain format method on string objects, effectively bypassing the sandbox's protections against format string attacks.

Once the sandbox is bypassed, an attacker can leverage Python's introspection capabilities to access arbitrary classes and execute code. This is particularly dangerous in web applications that allow users to create or modify templates, content management systems, email templating services, or any application that processes untrusted template input.

Root Cause

The root cause is classified under CWE-1336 (Improper Neutralization of Special Elements Used in a Template Engine). The |attr filter performed attribute lookups without going through the sandboxed environment's attribute access controls. This meant that while direct access to str.format was blocked by the sandbox, using |attr to access the same method was not intercepted, creating a bypass path.

Attack Vector

The attack requires local access with the ability to control template content. An attacker must be able to inject or modify template code that will be processed by the Jinja engine. While this requires some level of prior access or a specific application configuration that allows user-supplied templates, successful exploitation results in arbitrary code execution with the privileges of the application processing the template.

The exploitation technique involves crafting a template that uses the |attr filter to obtain a reference to str.format, then leveraging Python's format string capabilities and object introspection to access and execute arbitrary Python code. The attack chain typically involves navigating through Python's object hierarchy to find classes that provide code execution capabilities such as os.system or similar functions.

Detection Methods for CVE-2025-27516

Indicators of Compromise

• Unusual template content containing the |attr filter combined with format string patterns
• Template errors or exceptions related to attribute access or format operations
• Unexpected process spawning from Python web application processes
• Log entries showing template rendering failures with suspicious attribute chains

Detection Strategies

• Monitor template rendering logs for patterns involving |attr filter usage with string format methods
• Implement application-level logging for all template compilation and execution events
• Deploy runtime application self-protection (RASP) solutions that can detect sandbox escape attempts
• Review web application logs for error patterns indicative of exploitation attempts

Monitoring Recommendations

• Enable verbose logging for Jinja template processing in development and staging environments
• Monitor for unexpected child processes spawned by your web application server
• Set up alerts for template rendering exceptions that include attribute access patterns
• Implement file integrity monitoring on template directories to detect unauthorized modifications

How to Mitigate CVE-2025-27516

Immediate Actions Required

• Upgrade Jinja to version 3.1.6 or later immediately
• Audit all template sources to ensure untrusted content cannot be injected
• Review application architecture to minimize exposure of template processing to user input
• Implement additional input validation for any user-controllable template content

Patch Information

The vulnerability is fixed in Jinja version 3.1.6. The fix ensures that the |attr filter no longer bypasses the environment's attribute lookup, making sandbox protections consistent across all attribute access methods.

For detailed information about the fix, refer to the GitHub Commit Details and the GitHub Security Advisory.

Debian users should apply updates as documented in the Debian LTS Announcement.

Workarounds

• If immediate patching is not possible, disable or restrict the use of the |attr filter in templates through custom sandbox policies
• Implement strict content security policies that prevent user-supplied data from being processed as template code
• Use allowlisting for template sources, ensuring only trusted templates are processed
• Consider running template processing in isolated environments with minimal privileges

# Upgrade Jinja to patched version
pip install --upgrade jinja2>=3.1.6

# Verify installed version
pip show jinja2 | grep Version