Account impersonation via unsigned encrypted assertion selection

SAML validation in Red Hat build of Keycloak 26.2 before 26.2.14 and 26.4 before 26.4.10 allows remote authenticated users to impersonate principals. The SAML broker endpoint validates a signed plaintext assertion but decrypts and processes the first encrypted assertion without binding its signature when the response root is unsigned. Exploitation requires a valid assertion-level signature for the attacker's identity and a crafted response that places an encrypted assertion for the target principal first, producing an unauthorized brokered login and access to that principal's data.