How Can I Safely Pass a Table Name to a Stored Procedure?

Safely passing table names to stored procedures: striking a balance between dynamism and security

In the field of database programming, the ability to pass table names as parameters to stored procedures is crucial to achieve dynamic and flexible data operations. However, this task can have security implications, as poorly implemented code can lead to SQL injection attacks. This article explores an elegant and safe way to solve this problem.

Difficulty: Mixing Code and SQL Modifications

A common practice is to modify the code in large SQL statements based on user input. This approach is problematic because it allows user-supplied data to directly affect SQL queries, creating a potential vulnerability for SQL injection.

A safer way: parameterized stored procedures

A safer and more efficient alternative is to use parameterized stored procedures. Stored procedures are precompiled database objects that accept parameters, allowing you to pass user input as parameters without changing the SQL itself. This eliminates the risk of SQL injection while providing the required flexibility.

Challenge: Dynamically determine table names

However, challenges arise when the table to be selected depends on user input. For example, if the two parameters are "FOO" and "BAR", the query must dynamically choose between "FOO_BAR" or another table.

Dynamic SQL and table lookups

To solve this problem, we use dynamic SQL in conjunction with table lookups. We do not include the passed table name directly in the SQL query, but use it to retrieve the actual table name from the reference table. This is a key safeguard against SQL injection, as user-supplied data cannot be directly accessed by the executing query.

A simple example

Consider the following stored procedure:

CREATE PROC spCountAnyTableRows( @PassedTableName as NVarchar(255) ) AS
-- 安全地计算任何非系统表中的行数
BEGIN
    DECLARE @ActualTableName AS NVarchar(255)

    SELECT @ActualTableName = QUOTENAME( TABLE_NAME )
    FROM INFORMATION_SCHEMA.TABLES
    WHERE TABLE_NAME = @PassedTableName

    DECLARE @sql AS NVARCHAR(MAX)
    SELECT @sql = 'SELECT COUNT(*) FROM ' + @ActualTableName + ';'

    EXEC(@SQL)
END

This process dynamically constructs a SQL query based on the passed table name, ensuring that only rows from legitimate tables are returned.

Vulnerability Mitigation: Understanding the "Little Bobby Watch"

The famous XKCD comic "Little Bobby Table" illustrates the potential dangers of SQL injection. By cleverly embedding special characters in table names, an attacker can manipulate queries to access sensitive data or perform unauthorized operations. The table lookup in our example effectively prevents this type of attack because it ensures that user input cannot affect the actual table name used in the query.

Conclusion

Passing table names to stored procedures requires careful consideration of security implications. By combining dynamic SQL with table lookups, we create a powerful and flexible solution that eliminates the risk of SQL injection while maintaining the required dynamism.

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