Improper Validation of Function Hook Arguments

Description

Improper Validation of Function Hook Arguments occurs when an application allows external code to register hooks or callbacks but doesn't properly validate the arguments passed to or from these hooks. This is common in plugin systems, event-driven architectures, and extensible frameworks. Malicious hooks can receive sensitive data they shouldn't access, or return crafted values that corrupt application state.

Risk

Hooks can intercept and modify sensitive data in transit. Malicious plugins can escalate privileges through hook manipulation. Return values from hooks may corrupt security checks. Hooks can leak information to unauthorized parties. Denial of service through hooks that hang or crash. Supply chain attacks through compromised plugins.

Solution

Validate all data passed to hooks. Sanitize return values from hooks. Implement hook permissions system. Use typed interfaces for hook contracts. Sandbox hook execution where possible. Audit and verify plugin sources. Limit hook access to necessary data only.

Common Consequences

Impact	Details
Confidentiality	Scope: Information Disclosure Hooks can access sensitive data.
Integrity	Scope: Data Corruption Hook return values can corrupt state.
Authentication	Scope: Bypass Hooks can modify authentication flow.

Example Code + Solution Code

Vulnerable Code

<?php
// VULNERABLE: Hooks receive sensitive data
class VulnerableUserService {
    private array $hooks = [];

    public function registerHook(string $event, callable $callback): void {
        $this->hooks[$event][] = $callback;
    }

    public function authenticate(string $username, string $password): bool {
        // Hooks receive plaintext password!
        $this->triggerHook('before_auth', [
            'username' => $username,
            'password' => $password  // Sensitive!
        ]);

        $result = $this->doAuth($username, $password);

        // Hooks can modify result
        $this->triggerHook('after_auth', [
            'result' => &$result,  // Mutable reference!
            'user' => $this->getUser($username)
        ]);

        return $result;
    }

    private function triggerHook(string $event, array $args): void {
        foreach ($this->hooks[$event] ?? [] as $callback) {
            $callback($args);  // No validation!
        }
    }
}

// Malicious plugin
$service->registerHook('before_auth', function($args) {
    // Steals password
    file_put_contents('/tmp/passwords.txt',
        $args['username'] . ':' . $args['password'] . "\n",
        FILE_APPEND);
});

$service->registerHook('after_auth', function(&$args) {
    // Always grants access
    $args['result'] = true;
});
?>

# VULNERABLE: Django signal with sensitive data
from django.dispatch import receiver
from django.contrib.auth.signals import user_logged_in

@receiver(user_logged_in)
def vulnerable_login_handler(sender, request, user, **kwargs):
    # Any plugin can register and receive:
    # - Full user object
    # - Request with session
    # - All login details
    pass

# VULNERABLE: Flask hook with modifiable context
from flask import Flask, g

app = Flask(__name__)

@app.before_request
def vulnerable_before_request():
    # Plugins can modify g (application context)
    # which affects all subsequent processing
    pass

# Malicious plugin sets:
# g.user = admin_user  # Privilege escalation

// VULNERABLE: Event system without validation
class VulnerableEventSystem {
    constructor() {
        this.listeners = {};
    }

    on(event, callback) {
        this.listeners[event] = this.listeners[event] || [];
        this.listeners[event].push(callback);
    }

    emit(event, data) {
        const listeners = this.listeners[event] || [];

        // Each listener can modify data
        for (const listener of listeners) {
            listener(data);  // Data passed by reference
        }

        return data;
    }
}

// Usage
const events = new VulnerableEventSystem();

// Malicious plugin
events.on('user.authenticate', (data) => {
    // Modify authentication result
    data.authenticated = true;
    data.role = 'admin';
});

// Application code
const result = events.emit('user.authenticate', {
    username: 'attacker',
    authenticated: false
});

// result.authenticated is now true!

Fixed Code

<?php
// SAFE: Hooks with validated arguments
class SafeUserService {
    private array $hooks = [];
    private array $hookPermissions = [];

    public function registerHook(
        string $event,
        callable $callback,
        string $pluginId
    ): void {
        // Verify plugin is authorized for this hook
        if (!$this->canAccessHook($pluginId, $event)) {
            throw new UnauthorizedException("Plugin cannot access $event");
        }

        $this->hooks[$event][] = [
            'callback' => $callback,
            'plugin' => $pluginId
        ];
    }

    public function authenticate(string $username, string $password): bool {
        // Hooks receive only necessary, non-sensitive data
        $hookData = new AuthHookData(
            username: $username,
            timestamp: time()
            // NO password passed!
        );

        $this->triggerHook('before_auth', $hookData);

        $result = $this->doAuth($username, $password);

        // Result hooks receive immutable data
        $resultData = new AuthResultData(
            username: $username,
            success: $result,
            timestamp: time()
        );

        // Hooks cannot modify result directly
        $this->triggerHook('after_auth', $resultData);

        return $result;  // Original result unchanged
    }

    private function triggerHook(string $event, object $data): void {
        foreach ($this->hooks[$event] ?? [] as $hook) {
            try {
                // Clone data to prevent modification
                $hookData = clone $data;

                // Execute with timeout
                $this->executeWithTimeout(
                    fn() => $hook['callback']($hookData),
                    timeout: 1000
                );
            } catch (Throwable $e) {
                // Log but don't let hook crash application
                $this->logHookError($hook['plugin'], $event, $e);
            }
        }
    }
}

// Immutable hook data classes
readonly class AuthHookData {
    public function __construct(
        public string $username,
        public int $timestamp
    ) {}
}

readonly class AuthResultData {
    public function __construct(
        public string $username,
        public bool $success,
        public int $timestamp
    ) {}
}
?>

# SAFE: Django signals with filtered data
from django.dispatch import Signal

# Define signal with documented data contract
user_authenticated = Signal()  # Provides: user_id, timestamp only

class SafeAuthBackend:
    def authenticate(self, request, username, password):
        user = self._do_auth(username, password)

        if user:
            # Send only safe, non-sensitive data
            user_authenticated.send(
                sender=self.__class__,
                user_id=user.id,  # Not full user object
                timestamp=timezone.now()
                # NO request, NO session, NO password
            )

        return user

# SAFE: Flask with validated hooks
from flask import Flask, g
from functools import wraps

app = Flask(__name__)

class HookRegistry:
    def __init__(self):
        self.hooks = {}
        self.validators = {}

    def register(self, event, validator=None):
        def decorator(f):
            self.hooks.setdefault(event, []).append(f)
            return f
        return decorator

    def trigger(self, event, data):
        # Create immutable copy
        safe_data = self._sanitize(event, data)

        for hook in self.hooks.get(event, []):
            try:
                # Hooks cannot return values that affect flow
                hook(safe_data)
            except Exception as e:
                app.logger.error(f"Hook error: {e}")

    def _sanitize(self, event, data):
        # Remove sensitive fields
        sanitized = {k: v for k, v in data.items()
                    if k not in ['password', 'token', 'secret']}
        return types.MappingProxyType(sanitized)  # Immutable

// SAFE: Event system with validation
class SafeEventSystem {
    constructor() {
        this.listeners = new Map();
        this.schemas = new Map();
    }

    // Define expected data schema for event
    defineEvent(event, schema) {
        this.schemas.set(event, schema);
    }

    on(event, callback, pluginId) {
        // Verify plugin permissions
        if (!this.canListen(pluginId, event)) {
            throw new Error(`Plugin ${pluginId} cannot listen to ${event}`);
        }

        const listeners = this.listeners.get(event) || [];
        listeners.push({ callback, pluginId });
        this.listeners.set(event, listeners);
    }

    emit(event, data) {
        // Validate data against schema
        const schema = this.schemas.get(event);
        if (schema) {
            data = this.validateAndSanitize(data, schema);
        }

        // Create immutable copy
        const immutableData = Object.freeze({ ...data });

        const listeners = this.listeners.get(event) || [];

        for (const { callback, pluginId } of listeners) {
            try {
                // Pass copy, not reference
                callback({ ...immutableData });
            } catch (error) {
                console.error(`Hook error in ${pluginId}:`, error);
            }
        }

        // Return original data unchanged
        return data;
    }

    validateAndSanitize(data, schema) {
        const sanitized = {};

        for (const [key, type] of Object.entries(schema)) {
            if (key in data && typeof data[key] === type) {
                sanitized[key] = data[key];
            }
        }

        return sanitized;
    }
}

// Usage
const events = new SafeEventSystem();

events.defineEvent('user.authenticate', {
    username: 'string',
    timestamp: 'number'
    // No password field allowed
});

events.emit('user.authenticate', {
    username: 'user',
    timestamp: Date.now(),
    password: 'secret'  // Will be stripped
});

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Exploited in the Wild

WordPress Plugin Vulnerabilities

Malicious plugins exploited hook system for data theft.

Authentication Bypass

Hooks that could modify auth results led to bypasses.

Supply Chain Attacks

Compromised packages registered malicious hooks.

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Tools to test/exploit

• Plugin security scanners.

• Static analysis for hook vulnerabilities.

• Dynamic testing of event systems.

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CVE Examples

• CVEs from plugin/hook vulnerabilities in CMS platforms.

• Authentication bypass through hook manipulation.

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References

1. MITRE. "CWE-622: Improper Validation of Function Hook Arguments." https://cwe.mitre.org/data/definitions/622.html

2. Plugin security best practices documentation.