CVE-2026-25519: OpenSlides Authentication Bypass Flaw

CVE-2026-25519 Overview

CVE-2026-25519 is an authentication bypass vulnerability affecting OpenSlides, a free, web-based presentation and assembly system used for managing and projecting agenda, motions, and elections of an assembly. The vulnerability stems from incorrect access control in the local login mechanism for users synchronized via SAML single sign-on (SSO) from an external Identity Provider (IDP).

Prior to version 4.2.29, OpenSlides allows users to authenticate using either local credentials (username and password) or through an optionally configurable SAML-based SSO with an external IDP. The flaw allows attackers to bypass SAML authentication entirely by using the local login form with a SAML user's OpenSlides username and a trivial password that is valid for all SAML users.

Critical Impact

This authentication bypass allows unauthorized access to accounts of SAML-authenticated users, potentially compromising assembly management systems, confidential motions, and election data across organizations using OpenSlides with SAML integration.

Affected Products

• OpenSlides versions prior to 4.2.29
• OpenSlides Auth Service (affected component)
• OpenSlides deployments configured with SAML SSO integration

Discovery Timeline

• February 4, 2026 - CVE-2026-25519 published to NVD
• February 5, 2026 - Last updated in NVD database

Technical Details for CVE-2026-25519

Vulnerability Analysis

This vulnerability represents a critical flaw in the access control logic within OpenSlides' authentication service. When organizations configure OpenSlides to use SAML-based single sign-on, user accounts are synchronized from an external Identity Provider. However, the authentication service fails to properly differentiate between local user accounts and SAML-synchronized accounts during the local login process.

The core issue lies in how the authentication service handles login requests for SAML users. Instead of rejecting local login attempts for accounts that should exclusively authenticate through the external IDP, the system accepts a trivial password that works universally for all SAML users. This effectively renders the SAML security model useless, as any attacker who knows a SAML user's username can gain unauthorized access without needing to compromise the external IDP.

Root Cause

The root cause is improper access control (CWE-284) in the OpenSlides authentication service. The system fails to enforce that SAML-synchronized users must authenticate exclusively through the configured external Identity Provider. The authentication logic does not properly check the user's authentication source before validating local credentials, allowing a hardcoded or trivial password to successfully authenticate any SAML user account.

Attack Vector

The attack vector is network-based and does not require prior authentication or user interaction. An attacker can exploit this vulnerability through the following process:

1. The attacker identifies an OpenSlides instance configured with SAML SSO integration
2. The attacker enumerates or guesses valid usernames of SAML-synchronized users
3. Using the local login form, the attacker submits the target username with the trivial password
4. The authentication service incorrectly validates the credentials and grants access
5. The attacker gains full access to the victim's account with all associated privileges

The vulnerability requires high attack complexity as the attacker must identify SAML-configured instances and obtain valid usernames. However, once these prerequisites are met, exploitation is straightforward.

Detection Methods for CVE-2026-25519

Indicators of Compromise

• Successful local login attempts for user accounts that should only authenticate via SAML SSO
• Authentication logs showing local login activity for users who typically use external IDP authentication
• Unusual login patterns where SAML users appear to authenticate through the local login endpoint
• Multiple accounts accessing from the same IP address using local authentication when SAML is expected

Detection Strategies

• Monitor authentication logs for local login events associated with SAML-synchronized user accounts
• Implement alerting on authentication method mismatches where a user's expected authentication source differs from the method used
• Review access logs for sessions where the authentication method changed from SAML to local login
• Deploy anomaly detection to identify patterns of local authentication attempts across multiple SAML user accounts

Monitoring Recommendations

• Enable detailed authentication logging in OpenSlides to capture both successful and failed login attempts with authentication method details
• Configure SIEM rules to correlate authentication events and flag local logins for accounts provisioned through SAML
• Establish baseline authentication patterns for SAML users and alert on deviations
• Monitor the auth service component for unauthorized access patterns

How to Mitigate CVE-2026-25519

Immediate Actions Required

• Upgrade OpenSlides to version 4.2.29 or later immediately
• Review authentication logs for any suspicious local login activity involving SAML-synchronized accounts
• Force password resets and session invalidation for all SAML users as a precautionary measure
• Audit user accounts for any unauthorized changes made during the vulnerable period

Patch Information

OpenSlides has addressed this vulnerability in version 4.2.29. The fix is implemented in the authentication service component. Organizations should upgrade to the patched version as soon as possible.

For detailed information about the fix, refer to:

• OpenSlides Release 4.2.29
• GitHub Security Advisory GHSA-vv4h-8wfc-pf8c
• OpenSlides Auth Service Pull Request 889
• OpenSlides Auth Service Commit

Workarounds

• If immediate patching is not possible, consider temporarily disabling local login functionality for deployments using SAML authentication
• Implement network-level access controls to restrict access to the local login endpoint to trusted networks only
• Deploy a web application firewall (WAF) rule to monitor and potentially block suspicious authentication attempts
• Consider temporarily disabling SAML user accounts or the OpenSlides instance until the patch can be applied

# Configuration example
# After upgrading to 4.2.29, verify the version
docker exec openslides cat /app/VERSION
# Expected output: 4.2.29 or higher

# Review authentication logs for suspicious activity
docker logs openslides-auth-service | grep -i "local_login" | grep -i "saml_user"