Misskey CSS Style Injection Vulnerability In `MkUrlPreview`

published 2025-05-05 18 mo ago modified 2026-06-17 4 mo ago also EUVD-2025-13505

cvss: 7.2; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 17%
public exploits: 0; none known

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.

Patch / fix

fixed in 1 links 1

misskey misskey 2025.4.1

github.com/misskey-dev/misskey@d10fdfe commit

CISA SSVC

Exploitation: Proof of concept is public
Automatable: yes
Technical impact: partial
Assessed: 18 mo ago (2025-05-05)

Exploit prediction

0.26% chance of exploitation in the next 30 days, higher than 17% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	7.2	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
NVD	3.1	5.4	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

Affected products

misskey-dev misskey

Classification

Weakness

CWE-20 Improper Input Validation
CWE-116 Improper Encoding or Escaping of Output

References

Sources

CVE List CVE-2025-46340
NVD CVE-2025-46340

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary