Example 1
In 2008, a large number of web servers were compromised using the
same SQL injection attack string. This single string worked against many
different programs. The SQL injection was then used to modify the web sites
to serve malicious code. [1]
Example 2
The following code dynamically constructs and executes a SQL 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.
(Bad Code)
Example
Language: C#
...
string userName = ctx.getAuthenticatedUserName();
string query = "SELECT * FROM items WHERE owner = '" + userName +
"' AND itemname = '" + ItemName.Text + "'";
sda = new SqlDataAdapter(query, conn);
DataTable dt = new DataTable();
sda.Fill(dt);
...
The query that this code intends to execute follows:
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:
for itemName, then the query becomes the following:
SELECT * FROM items WHERE owner = 'wiley' AND itemname = 'name' OR
'a'='a';
The addition of the:
condition causes the WHERE clause to always evaluate to true, so the
query becomes logically equivalent to the much simpler query:
This simplification of the query allows the attacker to bypass the
requirement that the query 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 3
This example examines the effects of a different malicious value
passed to the query constructed and executed in the previous
example.
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:
(Attack)
Example
Language: SQL
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. 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 the previous example.
If an attacker enters the string
name'; DELETE FROM items; SELECT * FROM items WHERE 'a'='a
Then 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 SQL injection attacks is to
handle them as an input validation problem and either accept only
characters from a whitelist of safe values or identify and escape a
blacklist of potentially malicious values. Whitelisting can be a very
effective means of enforcing strict input validation rules, but
parameterized SQL statements require less maintenance and can offer more
guarantees with respect to security. As is almost always the case,
blacklisting is riddled with loopholes that make it ineffective at
preventing SQL injection attacks. For example, attackers can:
- Target fields that are not quoted
- Find ways to bypass the need for certain escaped
meta-characters
- Use stored procedures to hide the injected meta-characters.
Manually escaping characters in input to SQL queries can help, but it
will not make your application secure from SQL injection attacks.
Another solution commonly proposed for dealing with SQL injection
attacks is to use stored procedures. Although stored procedures prevent
some types of SQL injection attacks, they fail to protect against many
others. For example, the following PL/SQL procedure is vulnerable to the
same SQL injection attack shown in the first example.
procedure get_item ( itm_cv IN OUT ItmCurTyp, usr in varchar2, itm
in varchar2)
is open itm_cv for
' SELECT * FROM items WHERE ' || 'owner = '|| usr || ' AND
itemname = ' || itm || ';
end get_item;
Stored procedures typically help prevent SQL 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 SQL injection attacks.
Example 4
MS SQL has a built in function that enables shell command execution.
An SQL injection in such a context could be disastrous. For example, a query
of the form:
SELECT ITEM,PRICE FROM PRODUCT WHERE ITEM_CATEGORY='$user_input'
ORDER BY PRICE
Where $user_input is taken from the user and unfiltered.
If the user provides the string:
' exec master..xp_cmdshell 'vol' --
The query will take the following form: "
SELECT ITEM,PRICE FROM PRODUCT WHERE ITEM_CATEGORY='' exec
master..xp_cmdshell 'vol' --' ORDER BY PRICE
Now, this query can be broken down into:
[1] a first SQL query: SELECT ITEM,PRICE FROM PRODUCT WHERE
ITEM_CATEGORY=''
[2] a second SQL query, which executes a shell command: exec
master..xp_cmdshell 'vol'
[3] an MS SQL comment: --' ORDER BY PRICE
As can be seen, the malicious input changes the semantics of the query
into a query, a shell command execution and a comment.
Example 5
This code intends to print a message summary given the message
ID.
(Bad Code)
Example
Language: PHP
$id = $_COOKIE["mid"];
mysql_query("SELECT MessageID, Subject FROM messages WHERE
MessageID = '$id'");
The programmer may have skipped any input validation on $id under the
assumption that attackers cannot modify the cookie. However, this is
easy to do with custom client code or even in the web browser.
While $id is wrapped in single quotes in the call to mysql_query(), an
attacker could simply change the incoming mid cookie to:
This would produce the resulting query:
SELECT MessageID, Subject FROM messages WHERE MessageID = '1432'
or '1' = '1'
Not only will this retrieve message number 1432, it will retrieve all
other messages.
In this case, the programmer could apply a simple modification to the
code to eliminate the SQL injection:
(Good Code)
Example
Language: PHP
$id = intval($_COOKIE["mid"]);
mysql_query("SELECT MessageID, Subject FROM messages WHERE
MessageID = '$id'");
However, if this code is intended to support multiple users with
different message boxes, the code might also need an access control
check (CWE-285) to ensure that the application user has the permission
to see that message.
Example 6
This example attempts to take a last name provided by a user and
enter it into a database.
(Bad Code)
Example
Language: Perl
$userKey = getUserID();
$name = getUserInput();
# ensure only letters, hyphens and apostrophe are
allowed
$name = whiteList($name, "^a-zA-z'-$");
$query = "INSERT INTO last_names VALUES('$userKey',
'$name')";
While the programmer applies a whitelist to the user input, it has
shortcomings. First of all, the user is still allowed to provide hyphens
which are used as comment structures in SQL. If a user specifies -- then
the remainder of the statement will be treated as a comment, which may
bypass security logic. Furthermore, the whitelist permits the apostrophe
which is also a data / command separator in SQL. If a user supplies a
name with an apostrophe, they may be able to alter the structure of the
whole statement and even change control flow of the program, possibly
accessing or modifying confidential information. In this situation, both
the hyphen and apostrophe are legitimate characters for a last name and
permitting them is required. Instead, a programmer may want to use a
prepared statement or apply an encoding routine to the input to prevent
any data / directive misinterpretations.
Description
Weakness Class
Category
Weakness Variant
Weakness Base
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