Jinja2-2.10 Vulnerabilities: CVE-2019-10906 & CVE-2020-28493

In the realm of Python web development, Jinja2 stands out as a powerful and flexible template engine. However, like any software, it's crucial to stay informed about potential security vulnerabilities. This article delves into the vulnerabilities identified in Jinja2 version 2.10, specifically focusing on the Jinja2-2.10-py2.py3-none-any.whl package. We will explore the nature of these vulnerabilities, their potential impact, and the recommended steps for mitigation.

Vulnerable Library: Jinja2-2.10-py2.py3-none-any.whl

Jinja2 is a widely-used template engine written in pure Python. Its speed, ease of use, and standalone nature have made it a favorite among developers. This library allows developers to embed dynamic content within HTML, XML, or other markup formats. The specific library in question, Jinja2-2.10-py2.py3-none-any.whl, is a packaged version of Jinja2, compatible with both Python 2 and Python 3.

You can find more information about this library at its home page: https://files.pythonhosted.org/packages/7f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10-py2.py3-none-any.whl. However, it's important to note that this particular version has known vulnerabilities that should be addressed.

Findings: CVE-2019-10906 and CVE-2020-28493

Two significant vulnerabilities have been identified in Jinja2-2.10-py2.py3-none-any.whl:

• CVE-2019-10906: A high-severity vulnerability with a CVSS score of 8.6.
• CVE-2020-28493: A medium-severity vulnerability with a CVSS score of 5.3.

Let's delve into the details of each of these vulnerabilities.

CVE-2019-10906: Sandbox Escape via str.format_map

Vulnerability Details

This high-severity vulnerability, CVE-2019-10906, stems from an issue in Pallets Jinja versions prior to 2.10.1. Specifically, the str.format_map function allows for a sandbox escape. In the context of template engines, a sandbox is a restricted environment designed to prevent malicious code execution. A sandbox escape means that an attacker can bypass these restrictions and potentially execute arbitrary code on the server.

Think of it like this: imagine you're building a playground for children, but there's a hidden tunnel that allows them to leave the safe area and wander into a construction site. The str.format_map function in vulnerable versions of Jinja2 acted as that hidden tunnel, enabling attackers to bypass the intended security measures.

The vulnerability was published on April 6, 2019, and is documented in the Mend Vulnerability Database and the CVE database.

The threat assessment for this vulnerability is significant due to its potential for remote code execution. While the exploit maturity was not defined at the time of discovery, the EPSS score of 2.6000001% indicates a non-negligible probability of exploitation. This means that while not every system is likely to be attacked, the possibility is high enough to warrant immediate attention.

Suggested Fix

The recommended solution for CVE-2019-10906 is to upgrade to Jinja2 version 2.10.1 or later. This version includes a fix that addresses the vulnerability in str.format_map, effectively closing the sandbox escape. The fix was released on April 6, 2019, coinciding with the vulnerability's publication date. Upgrading is a straightforward process and is the most effective way to protect your application from this threat.

CVE-2020-28493: ReDoS Vulnerability in urlize Filter

Vulnerability Details

The second vulnerability, CVE-2020-28493, is a medium-severity issue related to a Regular Expression Denial of Service (ReDoS) attack. This vulnerability affects Jinja2 versions from 0.0.0 up to, but not including, 2.11.3. The root cause lies in the _punctuation_re regex operator used within the urlize filter. This filter is designed to automatically convert URLs in text to clickable links.

A ReDoS attack occurs when a poorly designed regular expression causes a program to consume excessive resources (CPU and memory) when processing specific input strings. In the case of Jinja2, the _punctuation_re regex, with its multiple wildcards, can become a bottleneck. The final wildcard, which searches for trailing punctuation, is particularly exploitable.

Imagine a scenario where an attacker crafts a malicious input string containing a long sequence of characters followed by specific punctuation. When the urlize filter processes this string, the regular expression engine can get stuck in a loop, attempting to match the pattern repeatedly. This can lead to a significant slowdown or even a complete denial of service.

This vulnerability was published on February 1, 2021, and is documented in the Mend Vulnerability Database and the CVE database.

The threat assessment for CVE-2020-28493 indicates a medium level of risk. While the exploit maturity is classified as