Nginx UI: Race Condition Leads to Persistent Data Corruption and Service Collapse

Nginx UI is a web user interface for the Nginx web server. Prior to version 2.3.4, 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. This issue has been patched in version 2.3.4.

Nginx UI is a web user interface for the Nginx web server. Prior to version 2.3.4, 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. This issue has been patched in version 2.3.4.

Nginx UI is a web user interface for the Nginx web server. Prior to version 2.3.4, 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. This issue has been patched in version 2.3.4.

### 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.

nginx-ui has Race Condition that Leads to Persistent Data Corruption and Service Collapse in github.com/0xJacky/Nginx-UI