It was found that Keycloak's SAML broker, versions from 1.9.0.CR1 up to 6.0.1, did not verify missing message signatures. If an attacker modifies the SAML Response and removes the <Signature> sections, the message is still accepted, and the message can be modified. An attacker could use this flaw to impersonate other users and gain access to sensitive information.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.