CVE-2026-57580: authentik: Account Takeover via SAML NameID Comment Truncation
authentik is an open-source identity provider. Prior to 2026.2.6 and 2026.5.5, an inbound SAML Source configured with the non-default USERNAMELINK or EMAILLINK user-matching mode interprets an XML comment in a NameID differently from the identity provider's signed assertion. An attacker with an account on the source identity provider who can set the account's NameID can inject an XML comment that truncates the value used by authentik to the text before the comment while the signed assertion remains valid. A crafted NameID can therefore truncate to a victim's username or email and bind the attacker's external identity to the victim's existing account. This grants full takeover without the victim's password or the identity provider's private key, and the malicious link persists so later logins succeed without the comment. Sources using the default unique-identifier matching mode and authentik's outbound SAML Provider role are not affected. This issue is fixed in versions 2026.2.6 and 2026.5.5.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
authentikto a version that resolves this vulnerability.Fixed in 2026.2.6 - Upgrade
Upgrade
authentikto a version that resolves this vulnerability.Fixed in 2026.5.5
Event History
Frequently Asked Questions
Which authentik SAML configurations are affected?
Deployments are exposed only when an inbound SAML Source uses the non-default USERNAME_LINK or EMAIL_LINK user-matching mode. Sources using the default unique-identifier matching mode and authentik deployments acting only as an outbound SAML Provider are not affected.
What access does an attacker need to exploit this issue?
An attacker needs an account at the source identity provider and the ability to set that account's NameID. They do not need the victim's password or the source identity provider's private key.
What should teams do if they cannot patch immediately?
Update authentik to 2026.2.6 or 2026.5.5. If an immediate update is not possible, use the default unique-identifier matching mode instead of USERNAME_LINK or EMAIL_LINK for inbound SAML Sources.
How can I check whether exploitation may have occurred?
A successful attack binds the attacker's external identity to an existing victim account, and that link persists after the malicious comment is removed. Review existing account links associated with affected inbound SAML Sources for unexpected external identities.