Craft CMS versions earlier than 5.10.11 contain a GraphQL entry mutation site scope bypass that allows an attacker holding a token scoped to a single site to read, modify, or delete entries in other sites by supplying an arbitrary siteId in mutation arguments. The flaw arises because ArgumentManager::prepareArguments() fails to validate the siteId, enabling a breach of confidentiality, integrity, and availability for content stored in the CMS. This is a classic instance of a logic flaw identified as CWE-639.

The affected product is Craft CMS, specifically community builds of craftcms:cms in the 5.0.0‑RC1 through 5.10.10 release line. Any installation running those version numbers is vulnerable until a patch is applied.

The vulnerability is remotely exploitable via the publicly exposed GraphQL endpoint, requiring only a valid API token with limited site scope; no additional privileges or host access are necessary. With a CVSS score of 8.7, the potential impact is high, though the EPSS score is currently unavailable, indicating no publicly known widespread exploitation yet. Because the flaw does not appear in the CISA KEV catalog, it is likely unexploited in the wild, but the easy path through a standard API call warrants immediate attention.