CVE-2026-100691 in Hugo
Summary
by MITRE • 09/26/2026
Hugo versions 0.75.0 through 0.165.x contain a stored cross-site scripting vulnerability: the syntax highlighter does not escape the `lineAnchors` option before passing it to Chroma, which writes the value verbatim into the `id` and `href` attributes of the generated line-number markup. A crafted `lineAnchors` value supplied as a Markdown code fence attribute (or passed to the `highlight` template function) results in unescaped HTML in the rendered page, allowing arbitrary JavaScript to execute in the browsers of visitors to the generated site. This affects sites that build and publish Markdown from untrusted contributors; Hugo's security model otherwise considers content trusted input. Fixed in 0.166.0, where the `lineAnchors` value is HTML-escaped before being passed to Chroma.
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Analysis
by VulDB Data Team • 09/26/2026
Hugo versions ranging from 0.75.0 through 0.165.x are susceptible to a stored cross-site scripting vulnerability within its syntax highlighting component. This flaw arises because the `lineAnchors` configuration option is not properly sanitized before being passed to the Chroma library, which handles code formatting and line numbering for rendered markdown content. When Hugo processes Markdown files containing code fences or utilizes the internal highlight template function, it incorporates the value of `lineAnchors` directly into the HTML structure without applying necessary escaping mechanisms. Specifically, this unescaped input is written verbatim into both the id attributes of span elements representing individual lines and the href attributes of anchor links associated with those lines. Consequently, if a malicious actor can control or influence the content being processed by Hugo, they can inject arbitrary JavaScript code that will be executed in the context of any user viewing the generated static site pages.
The operational impact of this vulnerability is significant for websites that accept Markdown input from untrusted sources, such as community forums, documentation sites with open contributions, or blogs allowing guest posts via markdown files. Since Hugo's default security model assumes content to be trusted and does not perform extensive sanitization on user-provided text by design, the lack of escaping in this specific technical path creates a critical attack vector. An attacker could craft a malicious `lineAnchors` value containing script tags or event handlers embedded within attribute values. When the site is built and served, these scripts execute automatically when visitors navigate to pages with highlighted code blocks that include line numbers. This execution occurs under the same origin policy as the website itself, allowing potential theft of session cookies, manipulation of page content, phishing attacks, or further propagation if combined with other vulnerabilities like server-side request forgery through crafted URLs in anchor tags.
This vulnerability is classified under Common Weakness Enumeration (CWE) ID 79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. More specifically, it aligns with CWE-80 due to the failure to escape special characters within HTML attributes before rendering them into the DOM structure. In terms of the MITRE ATT&CK framework for enterprise security, this flaw facilitates techniques associated with Client-side Injection and potentially Stored XSS attacks where malicious scripts are persisted on a server or static site generator output rather than being transient in URL parameters. The persistence aspect is critical here because the injected payload becomes part of the generated HTML files themselves, meaning every subsequent visitor to those pages triggers the execution without requiring additional interaction from the attacker beyond initial content submission.
Mitigation strategies primarily involve upgrading Hugo to version 0.166.0 or later, where developers have implemented proper HTML escaping for the `lineAnchors` value before it is passed to Chroma. For organizations unable to immediately upgrade their static site generation infrastructure, alternative mitigations include restricting input sources to only trusted contributors who do not attempt malicious injection, implementing strict Content Security Policy headers that restrict script execution origins if feasible within the deployment environment, or manually sanitizing any user-provided markdown content before it is processed by Hugo. Additionally, auditing existing generated sites for pre-existing injected payloads in line anchor attributes may be necessary if the vulnerability was exploited prior to patching. Ensuring that all dependencies and core tools are kept up-to-date remains the most effective defense against such implementation flaws in web generation frameworks.