Executive Summary

This vulnerability is an HTML injection issue in the mailgen Node.js package versions up to 2.0.31. When generating plaintext emails using the generatePlaintext method with user-supplied content, the code tries to remove HTML tags using a regular expression. However, it fails to remove tags that contain certain Unicode line separator characters. These tags are later decoded into valid HTML, allowing unexpected HTML content to remain in what should be plaintext output. If this output is rendered in a context that interprets HTML, it can lead to execution of attacker-supplied scripts in the victim's browser.

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Impact Analysis

This vulnerability can lead to execution of attacker-supplied scripts in the victim's browser if untrusted input is passed to mailgen's generatePlaintext method and the output is rendered in an HTML-interpreting context. This can result in cross-site scripting (XSS) attacks, potentially compromising user data, session tokens, or enabling other malicious actions within the affected application.

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Detection Guidance

You can detect this vulnerability by identifying if your project uses mailgen versions through 2.0.31 and calls the generatePlaintext method with untrusted input. Since the vulnerability involves HTML injection in plaintext emails, you can inspect generated plaintext emails for unexpected HTML content. There are no specific commands provided in the resources, but checking your package version with 'npm list mailgen' and reviewing code usage of 'generatePlaintext' with untrusted input is recommended.

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Mitigation Strategies

The immediate step to mitigate this vulnerability is to upgrade mailgen to version 2.0.32 or later, where the issue is fixed. Additionally, avoid passing untrusted input to the generatePlaintext method or ensure proper sanitization before usage.

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