This vulnerability in OpenSlides' authentication service allows attackers to bypass normal login checks for users synchronized from an external SAML identity provider. By entering any SAML user's username into the local login form and using a trivial password that is valid for all such users, an adversary can authenticate as that user without possessing the correct credentials or the IDP token. The flaw is a classic access‑control bypass (CWE‑284) that grants unauthorized access to the full capabilities of the target account, including viewing and casting votes, editing agendas, and other privileged functions within the assembly system.

The affected product is OpenSlides, a web‑based platform for managing assemblies. All installations running a version earlier than 4.2.29 are vulnerable because the fix was released in the 4.2.29 release. The issue is confined to the authentication module and does not affect other components directly, but any user with SAML credentials can be impersonated.

The CVSS score of 8.1 classifies the problem as High severity, and the EPSS score of less than 1 % indicates that, as of now, the probability of observed exploitation is low, although the flaw remains publicly documented. The vulnerability is not listed in CISA’s KEV catalog, reducing the likelihood of large‑scale active exploitation, but attackers could still carry out targeted credential‑guessing or social‑engineering strategies to obtain a valid SAML username. The attack capitalizes on the web login interface and requires no special privileges beyond interacting with the public authentication endpoint.