CVE-2025-46340: misskey Input Validation Flaw — Fix

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.

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CVSS v3.1 Score

7.2

HIGH

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

Weakness Type (CWE)

CWE-20 Improper Input Validation

CWE-116 CWE-116

Affected Products

Vendor	Product	Versions
misskey	misskey	12.0.0 — 2025.4.1

References

Advisories & Patches

https://github.com/misskey-dev/misskey/commit/d10fdfe9738b17a9d81037c031b40a2cc4cb8038 https://github.com/misskey-dev/misskey/security/advisories/GHSA-3p2w-xmv5-jm95

Frequently Asked Questions

What is CVE-2025-46340? +

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. It has a CVSS v3.1 base score of 7.2 (HIGH).

How severe is CVE-2025-46340? +

CVE-2025-46340 has a CVSS v3.1 score of 7.2 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-46340? +

CVE-2025-46340 affects products from misskey, specifically: misskey. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-46340? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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CVE-2025-46340