CVE-2026-61628 in Nginx Ignition
Summary
by MITRE • 09/21/2026
nginx ignition is a user interface for the nginx web server. Prior to version 2.41.1, `POST /api/users/onboarding/finish` is registered as anonymous (unauthenticated) and creates a user with full ReadWrite admin permissions. Because the handler uses a check-then-act (TOCTOU) pattern between the "onboarding already completed?" check and the user-creation write, with no atomic guard, a remote unauthenticated attacker who can reach an instance in its pre-onboarding state can create an administrator account for themselves — and concurrent requests can create multiple admin accounts in a single race. Version 2.41.1 patches the issue.
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Analysis
by VulDB Data Team • 09/21/2026
The vulnerability identified in nginx ignition prior to version 2.41.1 represents a critical authentication bypass resulting from improper handling of initial setup procedures within the application's user management subsystem. The specific flaw resides in the API endpoint POST /api/users/onboarding/finish, which is designed to finalize the onboarding process for new installations. In versions preceding the patch, this endpoint was registered as anonymous and unauthenticated, allowing any remote actor with network access to interact with it without providing valid credentials. This design oversight effectively bypasses the fundamental security principle of authentication during a critical initialization phase, granting attackers direct access to administrative account creation mechanisms that should be restricted to authorized personnel or isolated from external networks entirely.
The technical root cause is a classic Time-of-Check-to-Time-of-Use (TOCTOU) race condition embedded within the application logic. The handler employs a non-atomic check-then-act pattern where it first verifies whether onboarding has already been completed and then proceeds to create a new user if the check indicates otherwise. Because there is no atomic guard or database-level constraint to prevent concurrent execution of this sequence, multiple simultaneous requests can pass the initial existence check before any single request commits the creation transaction. This allows an unauthenticated attacker to exploit the race condition by sending rapid, parallel POST requests to the endpoint. The system fails to serialize these operations correctly, leading to the successful creation of multiple administrator accounts in a single attack session rather than just one.
The operational impact of this vulnerability is severe due to the privileges granted upon account creation. The user created through this exploit is assigned full ReadWrite admin permissions by default. This grants the attacker complete control over the nginx ignition interface and, depending on how the application integrates with the underlying nginx web server configuration, potentially access to sensitive server configurations, logs, or proxy settings. An adversary leveraging this flaw can establish persistent backdoor accounts, modify system parameters, exfiltrate data, or disrupt service availability. The ability to create multiple admin accounts exacerbates the risk by making remediation more difficult and increasing the likelihood of detection evasion through redundant entry points.
This vulnerability aligns with CWE-284 Improper Access Control, as it involves unauthorized access resulting from insufficient restrictions on authenticated users, specifically during a setup phase that should be restricted. It also maps to ATT&CK technique T1078 Valid Accounts, where attackers use legitimate credentials or create new valid accounts to maintain persistence and escalate privileges within the target environment. The race condition aspect further correlates with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, highlighting the failure to properly manage concurrent access to shared state during critical operations.
To mitigate this vulnerability, organizations must immediately upgrade nginx ignition to version 2.41.1 or later, which addresses the atomicity issue and restricts access to the onboarding endpoint. In environments where upgrading is not immediately feasible, network-level controls should be implemented to block external access to the /api/users/onboarding/finish endpoint until the system has been properly initialized by an authorized administrator. Additionally, implementing strict rate limiting on API endpoints can help mitigate race conditions by reducing the volume of concurrent requests that attackers can send in a short timeframe. Regular security audits and code reviews focusing on atomicity checks during user creation processes are recommended to prevent similar flaws in future development cycles.