High [CVE-2026-15571] Predictable account-linking hash enables account takeover via malicious OIDC client

A flaw was found in the legacy client-initiated account-linking endpoint of Keycloak, a widely used open-source identity and access management solution. The mechanism used to protect the account-linking process from unauthorized requests relies on a hash that can be predicted by a malicious OIDC client.

By tricking a user into authenticating, an attacker-controlled client can forge a valid linking URL to connect the victim's account to an attacker's external identity. This results in a full account takeover, allowing the attacker to log in as the victim.

The Red Hat Product Security team has assessed the severity of this vulnerability as Important, given that it enables full account takeover through a predictable security hash. Successful exploitation allows an attacker to link an unauthorized identity to a victim's account and subsequently impersonate that user across the realm.

The vulnerability's root cause is the use of predictable session identifiers and client-known metadata in the construction of the account-linking CSRF protection hash. Weakness: CWE-341.

Affected Red Hat products: Red Hat build of Keycloak 26.6; Red Hat build of Keycloak 26.6.6. Red Hat lists Red Hat Data Grid 8; Red Hat JBoss Enterprise Application Platform Expansion Pack; Red Hat Single Sign-On 7 as not affected.

Red Hat fixing advisory: RHSA-2026:56524, RHSA-2026:56523.