CVE-2024-26151 in mjml-python
Summary
by MITRE • 02/22/2024
The `mjml` PyPI package, found at the `FelixSchwarz/mjml-python` GitHub repo, is an unofficial Python port of MJML, a markup language created by Mailjet. All users of `FelixSchwarz/mjml-python` who insert untrusted data into mjml templates unless that data is checked in a very strict manner. User input like `<script>` would be rendered as `` in the final HTML output. The attacker must be able to control some data which is later injected in an mjml template which is then send out as email to other users. The attacker could control contents of email messages sent through the platform. The problem has been fixed in version 0.11.0 of this library. Versions before 0.10.0 are not affected by this security issue. As a workaround, ensure that potentially untrusted user input does not contain any sequences which could be rendered as HTML.
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Analysis
by VulDB Data Team • 03/13/2024
The mjml Python package represents a critical security vulnerability that exposes applications relying on this library to potential code injection attacks through improper input sanitization. This vulnerability specifically affects the unofficial Python implementation maintained by FelixSchwarz at the mjml-python GitHub repository, which serves as a bridge between the MJML markup language and Python environments. The core issue stems from insufficient validation of user-supplied data when rendering MJML templates into HTML output, creating a pathway for malicious actors to inject harmful content that bypasses standard security measures.
The technical flaw manifests when untrusted user data enters the MJML template processing pipeline without adequate sanitization. In the vulnerable versions of the library, user input containing HTML-like constructs such as script tags would be rendered directly into the final HTML output, effectively bypassing the intended security boundaries of the template system. This behavior creates a server-side template injection vulnerability that allows attackers to manipulate the content of emails generated by the application, potentially leading to cross-site scripting attacks, data exfiltration, or unauthorized actions within the context of the email recipients' browsers. The vulnerability is classified as a variant of CWE-79 (Cross-site Scripting) and falls under the ATT&CK technique T1190 (Exploit Public-Facing Application) when exploited through email platforms.
The operational impact of this vulnerability extends beyond simple content manipulation, as it enables attackers to compromise email communication channels and potentially gain access to sensitive user information. When applications process user-generated content through the mjml library without proper input validation, attackers can craft malicious inputs that, when rendered, execute unintended code in the recipients' email clients. This risk is particularly severe in web applications that send automated emails, user-generated content platforms, or any system where email templates contain dynamic user data. The vulnerability affects all versions prior to 0.11.0, with version 0.10.0 and earlier being unaffected due to their different processing mechanisms.
The security fix implemented in version 0.11.0 addresses the root cause by introducing proper input sanitization and validation mechanisms within the template rendering process. Organizations should immediately upgrade to this patched version to eliminate the risk of exploitation. As a temporary mitigation measure, administrators should implement strict input validation that removes or escapes potentially dangerous sequences from user-supplied data before it enters the MJML template processing pipeline. This approach aligns with defensive programming practices and helps prevent similar vulnerabilities in other components of the application stack. The vulnerability demonstrates the importance of proper content sanitization in web applications and highlights the need for comprehensive security testing of third-party libraries that handle user input.