XSS via Unescaped Code Block Language in Hugo

Executive Summary

This vulnerability exists in Hugo, a static site generator, where the default code-block renderer from versions 0.60.0 to 0.163.3 wrote the Markdown code-fence language (info-string) directly into the HTML code element's attributes without proper HTML escaping.

Because the language string was not escaped, an attacker could include a quote and a script payload in the info-string, which would break out of the attribute context and inject a live script element into the page, leading to a cross-site scripting (XSS) vulnerability.

This issue was fixed in version 0.163.3 by escaping the language attribute to prevent such script injection.

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Impact Analysis

If you use a vulnerable version of Hugo and process untrusted Markdown content that includes malicious code-fence language strings, an attacker could inject and execute arbitrary JavaScript in the generated site.

This cross-site scripting (XSS) vulnerability can lead to unauthorized actions on behalf of users, theft of sensitive information such as cookies or session tokens, and compromise of site visitors' security.

However, if only trusted content is processed, the risk is minimal.

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Detection Guidance

This vulnerability involves unescaped Markdown code-fence language strings in Hugo's default code-block renderer, which can lead to cross-site scripting (XSS) if malicious input is processed.

To detect this vulnerability on your system, you can check the version of Hugo you are running. Versions from 0.60.0 up to 0.163.2 are vulnerable, while version 0.163.3 and later include the fix.

You can run the following command to check your Hugo version:

• hugo version

Additionally, to detect if your generated site contains unescaped code-fence language attributes that could be exploited, you can search the generated HTML files for suspicious code blocks containing script tags or unescaped quotes in the language attribute.

For example, you can use grep to find occurrences of <script> tags or suspicious attributes in your public site directory:

• grep -r '<script' public/
• grep -r 'language=".*[\"\'].*' public/

If you find such patterns in your generated HTML, it may indicate that the vulnerability is present and exploitable.

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Mitigation Strategies

The primary mitigation step is to upgrade Hugo to version 0.163.3 or later, where the vulnerability has been fixed by properly escaping the language attribute in code block rendering.

If upgrading immediately is not possible, avoid processing untrusted Markdown content that could contain malicious code-fence language strings.

Review your content sources and ensure that only trusted content is processed by Hugo to reduce the risk of exploitation.

After upgrading, rebuild your site to ensure the fix is applied and verify that no unescaped script elements appear in your generated HTML.

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