CVE-2025-46340 is a high-severity vulnerability affecting the open-source federated social media platform Misskey, specifically versions from 12.0.0 up to but not including 2025.4.1. The flaw arises from improper input validation and insufficient escaping of CSS in the components `UrlPreviewService` and `MkUrlPreview`. When Misskey attempts to generate URL previews, the `UrlPreviewService.wrap` method falls back to returning the original URL if the protocol is not recognized (i.e., not http or https). This fallback behavior can inadvertently expose user information, potentially de-anonymizing users. More critically, the `MkUrlPreview` component applies a CSS `background-image` property without escaping CSS content, allowing an attacker to inject arbitrary CSS styles. This CSS injection can be exploited to craft malicious payloads that manipulate the appearance of the preview element, including creating fake error messages or UI elements designed to deceive users into divulging sensitive information such as credentials. The vulnerability is rooted in CWE-20 (Improper Input Validation) and CWE-116 (Improper Encoding or Escaping of Output). The vulnerability does not require authentication or user interaction to be exploited, and the CVSS 3.1 base score is 7.2, reflecting a high severity with network attack vector, low attack complexity, no privileges required, and no user interaction needed. Although no known exploits are currently reported in the wild, the potential for phishing-like attacks and user deception is significant. The issue was patched in Misskey version 2025.4.1.

For European organizations using Misskey as a federated social media platform, this vulnerability poses several risks. The CSS injection can lead to UI manipulation, enabling attackers to conduct phishing attacks within the platform by displaying fake error messages or login prompts, potentially harvesting user credentials or other sensitive data. The fallback behavior that leaks original URLs with unsupported protocols can de-anonymize users, undermining privacy guarantees critical in Europe, especially under GDPR regulations. This could result in reputational damage, regulatory penalties, and loss of user trust. Given Misskey's federated nature, a compromised instance could propagate malicious content or misinformation across the network, amplifying the impact. Organizations relying on Misskey for internal or community communications may face confidentiality breaches and social engineering risks. The lack of authentication or user interaction requirements lowers the barrier for exploitation, increasing the threat landscape.

European organizations should immediately upgrade all Misskey instances to version 2025.4.1 or later, where the vulnerability is patched. Until upgrades are completed, administrators should consider disabling URL preview features or restricting the protocols accepted by `UrlPreviewService` to only safe protocols (http and https) to prevent fallback to unrecognized protocols. Implement Content Security Policy (CSP) headers that restrict inline styles and disallow unsafe CSS to mitigate CSS injection impact. Conduct user awareness training to recognize suspicious UI elements and phishing attempts within the platform. Regularly audit and monitor Misskey logs for unusual URL preview requests or anomalous CSS payloads. For organizations running public-facing instances, consider deploying Web Application Firewalls (WAF) with custom rules to detect and block suspicious URL preview payloads. Finally, review federation peers and restrict federation with untrusted or unknown instances to reduce exposure to malicious content propagation.