CVE-2021-25987 is a stored Cross-site Scripting (XSS) vulnerability affecting Hexo, a popular static site generator widely used for blogging and documentation. The vulnerability exists in Hexo versions from 0.0.1 up to 5.4.0, where the 'post body' and 'tags' fields do not properly sanitize malicious JavaScript code during the web page generation process. This flaw allows a local unprivileged attacker to inject arbitrary JavaScript code that gets embedded into the generated static pages. When these pages are viewed by end users, the malicious script executes in their browsers, potentially leading to session hijacking, credential theft, or other client-side attacks. The vulnerability is classified under CWE-79, indicating improper neutralization of input during web page generation. The CVSS 3.1 base score is 5.0 (medium severity), with vector AV:L/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N, meaning the attack requires local access, low attack complexity, no privileges, user interaction, and impacts confidentiality and integrity with a scope change. No known exploits in the wild have been reported, and no official patches are linked, suggesting that mitigation relies on manual sanitization or upgrading Hexo beyond version 5.4.0. This vulnerability is particularly relevant for environments where multiple users have local access to the Hexo installation or where contributors can submit content that is then published without adequate sanitization. Since Hexo generates static sites, the attack surface is limited to the generation phase and the users consuming the generated content.

For European organizations using Hexo to generate static websites, this vulnerability could lead to client-side attacks against site visitors, including employees, customers, or partners. The injected scripts could steal session cookies, redirect users to malicious sites, or perform actions on behalf of users, undermining confidentiality and integrity of user data. Although the attack requires local access to inject malicious content, in collaborative environments such as companies or open-source projects where multiple contributors have access to the Hexo environment, the risk increases. The scope change in the CVSS vector indicates that the vulnerability can affect components beyond the initially compromised system, potentially impacting the integrity of published content and the trustworthiness of the website. This could damage brand reputation and lead to compliance issues under GDPR if personal data is compromised. However, the lack of known exploits and the requirement for local access reduce the likelihood of widespread automated attacks. The impact on availability is negligible as the vulnerability does not affect site uptime or Hexo functionality.

European organizations should prioritize upgrading Hexo to versions later than 5.4.0 where this vulnerability is addressed or confirmed fixed. If upgrading is not immediately possible, implement strict input validation and sanitization on all user-generated content, especially in the 'post body' and 'tags' fields, before site generation. Employ Content Security Policy (CSP) headers on the generated sites to restrict execution of unauthorized scripts and mitigate the impact of any injected code. Limit local access to Hexo environments to trusted users only, enforcing strict access controls and monitoring for unauthorized content changes. Use code review and automated scanning tools to detect potential XSS payloads in content before publishing. Additionally, educate contributors about secure content practices to avoid accidental injection of malicious scripts. Regularly audit generated site content for suspicious scripts or anomalies. Finally, consider isolating the Hexo build environment in a sandbox or container to reduce the risk of local exploitation.