SQL Injection: LINQ

Constructing a dynamic LINQ statement with input coming from an untrusted source might allow an attacker to modify the statement's meaning or to execute arbitrary SQL commands.

Injection errors related to LINQ occur when:

1. Data enters a program from an untrusted source.

2. The data is used to dynamically construct a query.
Example 1: The following code dynamically constructs and executes an LINQ query that searches for items matching a specified name. The query restricts the items displayed to those where owner matches the user name of the currently-authenticated user.

...
string userName = ctx.getAuthenticatedUserName();
string query = "SELECT * FROM items WHERE owner = '"
				+ userName + "' AND itemname = '"
				+ ItemName.Text + "'";

var items = dataContext.ExecuteCommand<Item>(query);
...

The query intends to execute the following code:

	SELECT * FROM items
	WHERE owner = <userName>
	AND itemname = <itemName>;

However, because the query is constructed dynamically by concatenating a constant base query string and a user input string, the query only behaves correctly if itemName does not contain a single-quote character. If an attacker with the user name wiley enters the string "name' OR 'a'='a" for itemName, then the query becomes the following:

	SELECT * FROM items
	WHERE owner = 'wiley'
	AND itemname = 'name' OR 'a'='a';

The addition of the OR 'a'='a' condition causes the where clause to always evaluate to true, so the query becomes logically equivalent to the much simpler query:

	SELECT * FROM items;

This simplification of the query allows the attacker to bypass the requirement that the query must only return items owned by the authenticated user. The query now returns all entries stored in the items table, regardless of their specified owner.

Example 2: This example examines the effects of a different malicious value passed to the query constructed and executed in Example 1. If an attacker with the user name wiley enters the string "name'); DELETE FROM items; --" for itemName, then the query becomes the following two queries:

	SELECT * FROM items
	WHERE owner = 'wiley'
	AND itemname = 'name';

	DELETE FROM items;

	--'

Many database servers, including Microsoft(R) SQL Server 2000, allow multiple SQL statements separated by semicolons to be executed at once. While this attack string results in an error on Oracle and other database servers that do not allow the batch-execution of statements separated by semicolons, on databases that do allow batch execution, this type of attack allows the attacker to execute arbitrary commands against the database.

Notice the trailing pair of hyphens (--), which specifies to most database servers that the remainder of the statement is to be treated as a comment and not executed [4]. In this case the comment character serves to remove the trailing single-quote left over from the modified query. On a database where comments are not allowed to be used in this way, the general attack could still be made effective using a trick similar to the one shown in Example 1. If an attacker enters the string "name'); DELETE FROM items; SELECT * FROM items WHERE 'a'='a", the following three valid statements will be created:

	SELECT * FROM items
	WHERE owner = 'wiley'
	AND itemname = 'name';

	DELETE FROM items;

	SELECT * FROM items WHERE 'a'='a';

One traditional approach to preventing LINQ injection attacks is to handle them as an input validation problem and either accept only characters from an allow list of safe values or identify and escape a list of potentially malicious values (deny list). Checking an allow list can be a very effective means of enforcing strict input validation rules, but parameterized LINQ statements require less maintenance and can offer more guarantees with respect to security. As is almost always the case, implementing a deny list is riddled with loopholes that make it ineffective at preventing LINQ injection attacks. For example, attackers may:

- Target fields that are not quoted
- Find ways to bypass the need for certain escaped metacharacters
- Use stored procedures to hide the injected metacharacters

Manually escaping characters in input to LINQ queries can help, but it will not make your application secure from LINQ injection attacks.

Another solution commonly proposed for dealing with LINQ injection attacks is to use stored procedures. Although stored procedures prevent some types of LINQ injection attacks, they fail to protect against many others. Stored procedures typically help prevent LINQ injection attacks by limiting the types of statements that can be passed to their parameters. However, there are many ways around the limitations and many interesting statements that can still be passed to stored procedures. Again, stored procedures can prevent some exploits, but they will not make your application secure against LINQ injection attacks.