CVE-2026-33028: Nginx UI: Race Condition Leads to Persistent Data Corruption and Service Collapse

Summary The nginx-ui application is vulnerable to a Race Condition. Due to the complete absence of synchronization mechanisms (Mutex) and non-atomic file writes, concurrent requests lead to the severe corruption of the primary configuration file (app.ini). This vulnerability results in a persistent Denial of Service (DoS) and introduces a non-deterministic path for Remote Code Execution (RCE) through configuration cross-contamination.

Details The vulnerability exists because the settings update pipeline does not implement any synchronization primitives. When multiple requests reach the handler simultaneously: 1. Memory Corruption: ProtectedFill() modifies shared global singleton pointers without thread-safety, leading to inconsistent states in memory. 2. File Corruption: The underlying library (gopkg.in/ini.v1) performs direct overwrites. Concurrent write operations interleave at the OS level, resulting in app.ini files with empty leading lines, truncated fields, or partially overwritten configuration keys. 3. State Persistent Failure: Depending on which bytes are corrupted, the application either fails its "is-installed" check (redirecting to /install) or encounters a fatal error during boot/runtime that prevents the process from responding to any further requests.

Environment: - OS: Kali Linux 6.17.10-1kali1 (6.17.10+kali-amd64) - Application Version: nginx-ui v2.3.3 (513) e5da6dd (go1.26.0) - Deployment: Docker Container

PoC 0. Check original app.ini file valid state: <img width="524" height="367" alt="image" src="https://github.com/user-attachments/assets/d9688f76-7fe7-46ea-9eb9-c55bf40918a6" />

1. Log in to the nginx-ui dashboard. 2. Navigate to Preferences and update settings. Capture a POST /api/settings request and send it to Burp Suite Intruder. 3. Configure the attack with Null payloads (to test basic concurrency) or a Fuzzing list (to test data-driven corruption). 4. Set the Resource Pool to 20-50 concurrent requests. <img width="1188" height="776" alt="image" src="https://github.com/user-attachments/assets/403eef43-2bc6-4651-8802-15ddcb4f7631" />

5. Observation (In-flight corruption): Monitor the app.ini file. You will observe the file being written with empty leading lines or incomplete key-value pairs.

- <img width="1316" height="390" alt="image" src="https://github.com/user-attachments/assets/d99553f7-d253-4525-9b45-f59994e69180" /> ------------------------------------------------

- <img width="1368" height="709" alt="image" src="https://github.com/user-attachments/assets/7522ba29-39f1-4c22-88f2-8e859cdb1984" />

6. Observation (Recovery Failure): If the service redirects to /install, attempting to complete the setup again often fails because the underlying configuration state is too corrupted to be reconciled by the installer logic. 7. Observation (Total Service Collapse): When the corruption in app.ini becomes so severe, the Go runtime or the INI parser encounters a fatal error, causing the Nginx-UI service to stop responding entirely (Hard DoS).

<img width="1344" height="542" alt="image" src="https://github.com/user-attachments/assets/da4b99dc-ddce-4b79-b0bb-2d634bdd3bf7" />

8. Observation (Cross-Section Contamination): During testing, it was observed that sometimes INI sections become interleaved. For example, fields belonging to the [nginx] section (like ConfigDir or ReloadCmd) were erroneously written under the [webauthn] section. Example of corrupted output observed: [webauthn] RPDisplayName = RPID = RPOrigins = gDirWhiteList = ConfigDir = /etc/nginx ConfigPath = PIDPath = /run/nginx.pid SbinPath = TestConfigCmd = ReloadCmd = nginx -s reload RestartCmd = nginx -s stop StubStatusPort = 51820 ContainerName =

Impact This is a High security risk (CWE-362: Race Condition). - Integrity: Permanent corruption of application settings and system-level configuration. - Availability: High. The attack results in a persistent Denial of Service that cannot be recovered via the web UI. - Remote Code Execution (RCE) Risk: Since the application allows updating certain fields (like Node Name) and uses others as shell commands (like ReloadCmd or RestartCmd), the observed "cross-contamination" of INI values means an attacker could potentially force a user-controlled string into a command execution field. If ReloadCmd is overwritten with a malicious payload provided in another field, the next nginx reload will execute that payload. While highly impactful, this specific exploit path is non-deterministic and depends on the precise interleaving of thread execution, making targeted exploitation difficult.

Recommended Mitigation 1. Implement Mutex Locking: Wrap the ProtectedFill and settings.Save() calls in a sync.Mutex to serialize access to global settings. 2. Atomic File Writes: Implement a "write-then-rename" strategy. Write the new configuration to app.ini.tmp and use os.Rename() to replace the original file atomically, ensuring the configuration file is always in a valid state.

A patched version of nginx-ui is available at https://github.com/0xJacky/nginx-ui/releases/tag/v2.3.4.