Identifying and Mitigating SQL Injection in WordPress Plugins: A Case Study with Perfect Survey v1.5.1
Introduction
SQL injection vulnerabilities are a persistent threat in web application security, particularly in platforms like WordPress where plugins often handle dynamic user input, and where a single bug could lead to millions of websites being impacted.
In this post, we’ll examine an SQL injection vulnerability discovered by Vincenzo Migliano in Perfect Survey v1.5.1 back in 2021. This was fixed a while back in versions greater than v1.5.1. Let’s dig in.
Understanding the Vulnerability
Understanding the vulnerability in Perfect Survey v1.5.1 requires a look into how SQL injection attacks exploit insufficient input handling within database queries. When applications accept user input without proper sanitization or parameterization, they risk allowing attackers to manipulate SQL queries directly. This lack of filtering and control can lead to vulnerabilities like SQL injection.
The vulnerability stems from how the question_id parameter is handled within the plugin. Specifically, the parameter is directly concatenated into SQL queries, opening the door for SQL injection attacks. What makes it particularly nasty is that it can be triggered by an unauthenticated user.
In the file PerfectSurveyPostTypeModel.php, the following line demonstrates the vulnerability:
$question = $this->wpdb->get_row('SELECT * FROM ' . $this->sql_table_questions . ' WHERE question_id = ' . $question_id, ARRAY_A);Here, the $question_id variable is directly inserted into the SQL query. If an attacker manipulates this variable, they can alter the SQL command, potentially accessing or modifying sensitive data.
The Attempted Sanitization with prsv_input_fetch
The plugin attempts to sanitize input using the prsv_input_fetch function, which includes a switch statement designed to handle different data types:
function prsv_input_fetch($key, $default = 0, $type = 'int') {
$var = isset($_REQUEST[$key]) ? $_REQUEST[$key] : $default;
switch(gettype($var)) {
case "boolean": $var = boolval($var); break;
case "double": $var = doubleval($var); break;
case "float": $var = floatval($var); break;
case "string": $var = sanitize_text_field($var); break;
case "integer": $var = intval($var); break;
}
return $var;The function casts variables based on their detected type. However, PHP’s gettype function determines the type based on the current value, which can be manipulated by an attacker. For example, if an attacker passes a string that looks like an integer (1 OR 1=1), gettype might still return string, causing the function to apply sanitize_text_field, which is ineffective in this context, but why?
The sanitize_text_field function is designed to clean text for HTML output, not for SQL queries. It doesn’t escape characters in a way that prevents SQL injection, especially when numeric values are expected and concatenated without quotes.
In the SQL query, the $question_id is not enclosed in quotes. Therefore, even if sanitize_text_field removes some malicious characters, an attacker can craft input that remains effective for injection without needing quotes.
While the intention behind the switch statement is to sanitize input based on type, it is not an effective method for preventing SQL injection. Sanitization should be context-specific and, for SQL queries, should involve parameterized queries rather than generic input cleaning. That said, I think the approach has merit, just not in isolation.
Testing for SQL Injection
To verify the vulnerability, we can craft a Proof of Concept (PoC) query that injects SQL code via the question_id parameter.
http://localhost:80/wordpress/wp-admin/admin-ajax.php?action=get_question&question_id=1 AND (SELECT 4595 FROM (SELECT(SLEEP(5)))x)This URL attempts to inject a SQL command that will cause the database to pause for 5 seconds (SLEEP(5)). If the response is delayed by approximately 5 seconds, it confirms that the SQL injection is successful. For further confirmation, the timer can be changed to different values.
Please note: Perform this test only in a controlled environment where you have permission to test for vulnerabilities.
Fixing the Vulnerability
To address the vulnerability, you should use parameterized queries provided by WordPress’s $wpdb class, specifically the prepare method.
$question = $this->wpdb->get_row(
$this->wpdb->prepare('SELECT * FROM ' . $this->sql_table_questions . ' WHERE question_id = %d', $question_id),
ARRAY_A
);$wpdb->prepare safely escapes the input and ensures it is treated as an integer (%d), preventing any injected SQL code from being executed.
Conclusion
This case study highlights the critical importance of proper input handling and query parameterization in preventing SQL injection vulnerabilities. The attempted sanitization using a switch statement in Perfect Survey v1.5.1, although useful, was insufficient. Here were the key takeaways:
Sanitize Appropriately: Use sanitization functions that are appropriate for the context in which the data will be used. In this case, the switch statement attempts to handle multiple datatypes, but the datatype is determined by user-input and may lead to unexpected behavior.
Parameterize Queries: Always use parameterized queries for database interactions to prevent SQL injection. An example fix was provided above.
Understand Limitations of Sanitization Functions: Functions like sanitize_text_field are not substitutes for proper query parameterization.
Ongoing education and continuous testing enable developers to identify bugs early on, strengthening applications and reducing security risks before they reach end users.
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