77 lines
2.8 KiB
Markdown
77 lines
2.8 KiB
Markdown
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---
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title: Query Parameterization
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---
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## Query Parameterization
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A common mistake when connecting your program to a database is to accept a user's input and apply it directly to the database without checking it first. This is a dangerous habit to get into, and you may hear more experienced developers warning others to "sanitize input" or "parameterize queries".
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Let's start with a short example demonstrating the problem:
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_(the following snippets are written in C# for MySQL, but the concept applies to any language and database)_
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### The Problem
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```csharp
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public void RetrieveEmployeeInfo(string username)
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{
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using (var connection = new MySqlConnection("valid_connection_string"))
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{
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var query = "SELECT * FROM EMPLOYEES WHERE USERNAME = '" + username + "'";
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using (var command = new MySqlCommand(query, connection))
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{
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var reader = command.ExecuteReader();
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while (reader.Read())
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{
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// do something with the results of your query, like display the employee
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}
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}
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}
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}
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```
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At first glance, that might seem fairly harmless. If the user types "JDOE" into your program, and it's passed to this function, you'll end up executing a query like this:
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```sql
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SELECT * FROM EMPLOYEES WHERE USERNAME = 'JDOE';
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```
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The problem becomes more apparent when you consider what happens if the user _doesn't_ type what you expect. What if they type something like `JDOE'; DROP TABLE EMPLOYEES; --`? Your "query" string now looks like this, which will select the employee info, then delete the entire EMPLOYEES table!
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```sql
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SELECT * FROM EMPLOYEES WHERE USERNAME = 'JDOE'; DROP TABLE EMPLOYEES; --'
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```
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### The Solution
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To prevent issues like this, we can parameterize our queries. Let's look at another example:
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```csharp
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public void RetrieveEmployeeInfo(string username)
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{
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using (var connection = new MySqlConnection("valid_connection_string"))
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{
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var query = "SELECT * FROM EMPLOYEES WHERE USERNAME = @username";
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using (var command = new MySqlCommand(query, connection))
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{
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command.Parameters.AddWithValue("username", username);
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var reader = command.ExecuteReader();
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while (reader.Read())
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{
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// do something with the results of your query, like display the employee
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}
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}
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}
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}
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```
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Now what happens if the user types in `JDOE'; DROP TABLE EMPLOYEES; --`? Our program ends up executing a query like this one and, finding no employee whose username actually matches that input, simply returns no records.
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```sql
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SELECT * FROM EMPLOYEES WHERE USERNAME = 'JDOE\'; DROP TABLE EMPLOYEES; --'
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```
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No matter which language or database you're using, if you consider querying the database using user input, check the documentation for the proper way to parameterize queries.
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