This is an RCE vulnerability that affects Craft 4 and 5 installs where your security key has already been compromised.
https://craftcms.com/knowledge-base/securing-craft#keep-your-secrets-secret
Anyone running an unpatched version of Craft with a compromised security key is affected.
This has been patched in Craft 5.5.8 and 4.13.8.
If you can't update to a patched version, then rotating your security key and ensuring its privacy will help to migitgate the issue.
https://github.com/craftcms/cms/commit/e59e22b30c9dd39e5e2c7fe02c147bcbd004e603
{ "nvd_published_at": "2025-01-18T01:15:07Z", "cwe_ids": [ "CWE-94" ], "severity": "HIGH", "github_reviewed": true, "github_reviewed_at": "2025-01-21T19:48:38Z" }