A1 Injection

This page was based on OWASP reference materials.

A1.1 SQL Injection

Injection flaws, such as SQL, OS, and LDAP injection occur when untrusted data is sent to an interpreter as part of a command or query. The attacker’s hostile data can trick the interpreter into executing unintended commands or accessing data without proper authorization.

Primary Defenses:

SQL Injection Prevention Cheat Sheet

1. Prepared Statements (with Parameterized Queries)

The following code example uses a PreparedStatement, Java’s implementation of a parameterized query, to execute the same database query.

String custname = request.getParameter("customerName"); // This should REALLY be validated too
// perform input validation to detect attacks
String query = "SELECT account_balance FROM user_data WHERE user_name =&nbsp;? ";
PreparedStatement pstmt = connection.prepareStatement( query );*
pstmt.setString( 1, custname);*
ResultSet results = pstmt.executeQuery( );

1.1. Safe Java Prepared Statement ExampleThe following code example uses a PreparedStatement, Java's implementation of a parameterized query, to execute the same database query.

 String custname = request.getParameter("customerName"); // This should REALLY be validated too
// perform input validation to detect attacks
String query = "SELECT account_balance FROM user_data WHERE user_name =&nbsp;? ";
PreparedStatement pstmt = connection.prepareStatement( query );
pstmt.setString( 1, custname);
ResultSet results = pstmt.executeQuery( );

h5. 1.2. Hibernate Query Language (HQL) Prepared Statement (Named Parameters) Examples

First is an unsafe HQL Statement

 Query unsafeHQLQuery = session.createQuery("from Inventory where productID='"userSuppliedParameter"'");

Here is a safe version of the same query using named parameters:

Query safeHQLQuery = session.createQuery("from Inventory where productID=:productid");

 safeHQLQuery.setParameter("productid", userSuppliedParameter);

Defense Option 2: Stored Procedures

String custname = request.getParameter("customerName"); // This should REALLY be validated

 try {

                    *CallableStatement cs = connection.prepareCall("{call sp_getAccountBalance\(?)}");*

                    cs.setString(1, custname);

                    ResultSet results = cs.executeQuery();                             

                    // ... result set handling

 } catch (SQLException se)

Defense Option 3: White List Input Validation

Various parts of SQL queries aren't legal locations for the use of bind variables, such as the names of tables or columns, and the sort order indicator (ASC or DESC). In such situations, input validation or query redesign is the most appropriate defense. For the names of tables or columns, ideally those values come from the code, and not from user parameters. But if user parameter values are used to make different for table names and column names, then the parameter values should be mapped to the legal/expected table or column names to make sure unvalidated user input doesn't end up in the query. Please note, this is a symptom of poor design and a full re-write should be considered if time allows. Here is an example of table name validation.

 String tableName;
switch(PARAM):
case "Value1": tableName = "fooTable";
break;
case "Value2": tableName = "barTable";
break;
...
default     &nbsp;: throw new InputValidationException("unexpected value provided for table name");

The tableName can then be directly appended to the SQL query since it is now known to be one of the legal and expected values for a table name in this query. Keep in mind that generic table validation functions can lead to data loss as table names are used in queries where they are not expected.

For something simple like a sort order, it would be best if the user supplied input is converted to a boolean, and then that boolean is used to select the safe value to append to the query. This is a very standard need in dynamic query creation. For example:

 public String someMethod(boolean sortOrder) {

 String SQLquery = "some SQL ... order by Salary " + (sortOrder ? "ASC" : "DESC");
 ...

Any time user input can be converted to a non-String, like a date, numeric, boolean, enumerated type, etc. before it is appended to a query, or used to select a value to append to the query, this ensures it is safe to do so.

Input validation is also recommended as a secondary defense in ALL cases, even when using bind variables as is discussed later in this article. More techniques on how to implement strong white list input validation is described in the Input Validation Cheat Sheet.

Defense Option 4: Escaping All User Supplied Input

This second technique is to escape user input before putting it in a query. However, this methodology is frail compared to using parameterized queries and we cannot guarantee it will prevent all SQL Injection in all situations. This technique should only be used, with caution, to retrofit legacy code in a cost effective way. Applications built from scratch, or applications requiring low risk tolerance should be built or re-written using parameterized queries.

This technique works like this. Each DBMS supports one or more character escaping schemes specific to certain kinds of queries. If you then escape all user supplied input using the proper escaping scheme for the database you are using, the DBMS will not confuse that input with SQL code written by the developer, thus avoiding any possible SQL injection vulnerabilities.

Secure Coding - Encode and Escape

https://www.securecoding.cert.org/confluence/display/java/IDS51-J.+Properly+encode+or+escape+output3.1 Use ESAPI database encoders for:

https://owasp-esapi-java.googlecode.com/svn/trunk_doc/latest/org/owasp/esapi/codecs/Codec.html

3.2 Escaping Dynamic Queries: Turn off character replacement

The LIKE keyword allows for text scanning searches. In Oracle, the underscore '_' character matches only one character, while the ampersand '%' is used to match zero or more occurrences of any characters. These characters must be escaped in LIKE clause criteria. For example:

SELECT name FROM emp
WHERE id LIKE '%/_%' ESCAPE '/';
SELECT name FROM emp
WHERE id LIKE '%%%' ESCAPE '\';

* *SQL Injection Best Practices

Least Privilege

To minimize the potential damage of a successful SQL injection attack, you should minimize the privileges assigned to every database account in your environment. Do not assign DBA or admin type access rights to your application accounts. We understand that this is easy, and everything just ‘works’ when you do it this way, but it is very dangerous. Start from the ground up to determine what access rights your application accounts require, rather than trying to figure out what access rights you need to take away. Make sure that accounts that only need read access are only granted read access to the tables they need access to. If an account only needs access to portions of a table, consider creating a view that limits access to that portion of the data and assigning the account access to the view instead, rather than the underlying table. Rarely, if ever, grant create or delete access to database accounts.

If you adopt a policy where you use stored procedures everywhere, and don’t allow application accounts to directly execute their own queries, then restrict those accounts to only be able to execute the stored procedures they need. Don’t grant them any rights directly to the tables in the database.

SQL injection is not the only threat to your database data. Attackers can simply change the parameter values from one of the legal values they are presented with, to a value that is unauthorized for them, but the application itself might be authorized to access. As such, minimizing the privileges granted to your application will reduce the likelihood of such unauthorized access attempts, even when an attacker is not trying to use SQL injection as part of their exploit.

While you are at it, you should minimize the privileges of the operating system account that the DBMS runs under. Don't run your DBMS as root or system! Most DBMSs run out of the box with a very powerful system account. For example, MySQL runs as system on Windows by default! Change the DBMS's OS account to something more appropriate, with restricted privileges.

Multiple DB Users

The designer of web applications should not only avoid using the same owner/admin account in the web applications to connect to the database. Different DB users could be used for different web applications. In general, each separate web application that requires access to the database could have a designated database user account that the web-app will use to connect to the DB. That way, the designer of the application can have good granularity in the access control, thus reducing the privileges as much as possible. Each DB user will then have select access to what it needs only, and write-access as needed.

As an example, a login page requires read access to the username and password fields of a table, but no write access of any form (no insert, update, or delete). However, the sign-up page certainly requires insert privilege to that table; this restriction can only be enforced if these web apps use different DB users to connect to the database.

Views

SQL views can further increase the granularity of access by limiting the read access to specific fields of a table or joins of tables. It could potentially have additional benefits: for example, suppose that the system is required (perhaps due to some specific legal requirements) to store the passwords of the users, instead of salted-hashed passwords. The designer could use views to compensate for this limitation; revoke all access to the table (from all DB users except the owner/admin) and create a view that outputs the hash of the password field and not the field itself. Any SQL injection attack that succeeds in stealing DB information will be restricted to stealing the hash of the passwords (could even be a keyed hash), since no DB user for any of the web applications has access to the table itself.

Java applications using XML libraries are particularly vulnerable to XXE because the default settings for most Java XML parsers is to have XXE enabled. To use these parsers safely, you have to explicitly disable XXE in the parser you use. The following describes how to disable XXE in the most commonly used XML parsers for Java.

A 1.2 XML External Entity (XXE)

An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts. The following guide provides concise information to prevent this vulnerability. For more information on XXE, please visit XML External Entity (XXE) Processing.

A 1.2. Best Practices

The safest way to prevent XXE is always to disable DTDs (External Entities) completely. Depending on the parser, the method should be similar to the following:factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);Disabling DTDs also makes the parser secure against denial of services (DOS) attacks such as Billion Laughs. If it is not possible to disable DTDs completely, then external entities and external doctypes must be disabled in the way that’s specific to each parser.

Detailed XXE Prevention guidance for a number of languages and commonly used XML parsers in those languages is provided below.

JAXP DocumentBuilderFactory and SAXParserFactory

Both DocumentBuilderFactory and SAXParserFactory XML Parsers can be configured using the same techniques to protect them against XXE. Only the DocumentBuilderFactory example is presented here. The JAXP DocumentBuilderFactory setFeature method allows a developer to control which implementation-specific XML processor features are enabled or disabled. The features can either be set on the factory or the underlying XMLReader setFeature method. Each XML processor implementation has its own features that govern how DTDs and external entities are processed.

For a syntax highlighted code snippet for DocumentBuilderFactory, click here.

For a syntax highlighted code snippet for SAXParserFactory, click here.import javax.xml.parsers.DocumentBuilderFactory;

import javax.xml.parsers.ParserConfigurationException; // catching unsupported features
...

    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    try {
      // This is the PRIMARY defense. If DTDs (doctypes) are disallowed, almost all XML entity attacks are prevented
      // Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl
      String FEATURE = "http://apache.org/xml/features/disallow-doctype-decl";
      dbf.setFeature(FEATURE, true);
 
      // If you can't completely disable DTDs, then at least do the following:
      // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities
      // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities
      // JDK7+ - http://xml.org/sax/features/external-general-entities   
      FEATURE = "http://xml.org/sax/features/external-general-entities";
      dbf.setFeature(FEATURE, false);
 
      // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-parameter-entities
      // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-parameter-entities
      // JDK7+ - http://xml.org/sax/features/external-parameter-entities   
      FEATURE = "http://xml.org/sax/features/external-parameter-entities";
      dbf.setFeature(FEATURE, false);

      // Disable external DTDs as well
      FEATURE = "http://apache.org/xml/features/nonvalidating/load-external-dtd"
      dbf.setFeature(FEATURE, false);
 
      // and these as well, per Timothy Morgan's 2014 paper: "XML Schema, DTD, and Entity Attacks" (see reference below)
      dbf.setXIncludeAware(false);
      dbf.setExpandEntityReferences(false);

      // And, per Timothy Morgan: "If for some reason support for inline DOCTYPEs are a requirement, then
      // ensure the entity settings are disabled (as shown above) and beware that SSRF attacks
      // (http://cwe.mitre.org/data/definitions/918.html) and denial
      // of service attacks (such as billion laughs or decompression bombs via "jar:") are a risk."
 
      // remaining parser logic
      ...


      catch (ParserConfigurationException e) {
            // This should catch a failed setFeature feature
            logger.info("ParserConfigurationException was thrown. The feature '" +
                        FEATURE +
                        "' is probably not supported by your XML processor.");
            ...
        }
      catch (SAXException e) {
            // On Apache, this should be thrown when disallowing DOCTYPE
            logger.warning("A DOCTYPE was passed into the XML document");
            ...
        }
     catch (IOException e) {
            // XXE that points to a file that doesn't exist
            logger.error("IOException occurred, XXE may still possible: " + e.getMessage());
            ...
        }

References:

https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet

A 1.3. ORM Mappers

In most cases, using ORM mapper such as Hibernate will protect you from SQL Injection since all database calls are implemented with prepared statements. In Hibernate, avoid code such as:

Query query = session.createQuery("SELECT * FROM TABLE WHERE SOMEVAL = ' + user_supplied_variable + "'");

Use binding syntax instead:

Query query = session.createQuery("SELECT * FROM TABLE WHERE SOMEVAL= :someval";
query.setString(":someval", user_supplied_variable);

A 1.4. LDAP Injection

Use positive validation to eliminate all but valid username and other dynamic inputs. The following code is vulnerable to LDAP injection:

String principal = "cn=" + getParameter("username") + ", ou=Users, o=example";
String password = getParameter("password");
env.put(Context.SECURITY_AUTHENTICATION, "simple");
env.put(Context.SECURITY_PRINCIPAL, principal);
env.put(Context.SECURITY_CREDENTIALS, password);
// Create the initial context
DirContext ctx = new InitialDirContext(env);

//Instead, implement code for LDAP as follows: 
//if the username contains LDAP specials, stop now
if ( containsLDAPspecials(getParameter("username")) ) {
    throw new javax.naming.AuthenticationException();
}
String principal = "cn=" + getParameter("username") + ", ou=Users, o=example";
String password = getParameter("password");
env.put(Context.SECURITY_AUTHENTICATION, "simple");
env.put(Context.SECURITY_PRINCIPAL, principal);
env.put(Context.SECURITY_CREDENTIALS, password);
// Create the initial context
DirContext ctx = new InitialDirContext(env);

Learn More:

Videos:

Basic SQL Injection:

http://www.youtube.com/watch?v=pypTYPaU7mM

https://www.youtube.com/watch?v=02mLrFVzIYU&list=PLoyY7ZjHtUUVLs2fy-ctzZDSPpawuQ28d

Advanced SQL by Joe McCray:

https://www.youtube.com/watch?v=rdyQoUNeXSg