Misskey is an open source, federated social media platform. Starting in version 12.0.0 and prior to version 2025.4.1, due to an oversight in the validation performed in UrlPreviewService and MkUrlPreview, it is possible for an attacker to inject arbitrary CSS into the MkUrlPreview component. UrlPreviewService.wrap falls back to returning the original URL if it's using a protocol that is likely to not be understood by Misskey, IE something other than http or https. This both can de-anonymize users and_allow further attacks in the client.

Additionally, MkUrlPreview doesn't escape CSS when applying a background-image property, allowing an attacker to craft a URL that applies arbitrary styles to the preview element. Theoretically, an attacker can craft a CSS injection payload to create a fake error message that can deceive the user into giving away their credentials or similar sensitive information. Version 2025.4.1 contains a patch for the issue.