Description

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.

Classification

CVE ID: CVE-2025-46340

CVSS Base Severity: HIGH

CVSS Base Score: 7.2

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Affected Products

Vendor: misskey-dev

Product: misskey

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.06% (probability of being exploited)

EPSS Percentile: 20.34% (scored less or equal to compared to others)

EPSS Date: 2025-06-03 (when was this score calculated)

Stakeholder-Specific Vulnerability Categorization (SSVC)

SSVC Exploitation: poc

SSVC Technical Impact: partial

SSVC Automatable: true

References

https://nvd.nist.gov/vuln/detail/CVE-2025-46340
https://github.com/misskey-dev/misskey/security/advisories/GHSA-3p2w-xmv5-jm95
https://github.com/misskey-dev/misskey/commit/d10fdfe9738b17a9d81037c031b40a2cc4cb8038

Timeline