CVE-2025-3859 is a medium-severity vulnerability affecting Mozilla Focus, a privacy-centric mobile web browser developed by Mozilla. The vulnerability arises from the way Firefox Focus handles the display of long URLs in the browser's address bar. Specifically, when URLs are excessively long, the browser truncates or 'elides' the URL in the location view to fit the display constraints. This truncation can be exploited by malicious websites to craft URLs that appear differently in the truncated view, potentially misleading users into believing they are visiting a legitimate or trusted webpage when they are not. This form of address bar spoofing is a type of user interface deception attack, categorized under CWE-451 (User Interface Misrepresentation). The vulnerability does not require any privileges or authentication to exploit but does require user interaction, such as clicking on a crafted link. The CVSS 3.1 base score is 4.3, reflecting a network attack vector with low complexity, no privileges required, user interaction needed, and limited impact on confidentiality only, with no integrity or availability impact. No known exploits have been reported in the wild as of the publication date. The affected versions are all versions of Mozilla Focus prior to version 138, though the exact affected versions are unspecified. The vulnerability was published on April 30, 2025, and remains unpatched as no patch links are provided. This vulnerability could be leveraged in phishing or social engineering campaigns to increase the likelihood of users divulging sensitive information or credentials by misrepresenting the true destination URL in the address bar.

For European organizations, this vulnerability poses a moderate risk primarily in the context of phishing and social engineering attacks. Since Mozilla Focus is a privacy-focused browser often used on mobile devices, users relying on it for secure browsing could be misled by the truncated URL display, increasing the risk of credential theft or exposure to malicious websites. This could impact sectors with high sensitivity to phishing, such as financial institutions, government agencies, and critical infrastructure operators. The confidentiality of user data could be compromised if users are tricked into submitting credentials or sensitive information to fraudulent sites. However, the vulnerability does not directly affect system integrity or availability, limiting the scope of damage to user trust and data confidentiality. The risk is heightened in environments where users are less aware of URL spoofing tactics or where security awareness training is insufficient. Additionally, the lack of a patch at the time of disclosure means organizations must rely on mitigation strategies until an official fix is released.

European organizations should implement targeted user awareness training focusing on the risks of URL truncation and address bar spoofing, emphasizing vigilance when clicking on links, especially from untrusted sources. Technical mitigations include encouraging users to verify URLs by long-pressing or copying the link to view the full address before visiting, or using alternative browsers that do not exhibit this truncation behavior until Mozilla releases a patch. Organizations should monitor Mozilla Focus usage within their environment and consider restricting or advising against its use for sensitive browsing activities until the vulnerability is patched. Additionally, deploying email and web filtering solutions that detect and block URLs with suspiciously crafted long addresses can reduce exposure. Security teams should stay updated on Mozilla’s advisories for the release of patches and promptly apply updates once available. Finally, multi-factor authentication should be enforced to mitigate the impact of credential theft resulting from phishing attempts leveraging this vulnerability.