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Command-Line Parameters Up through Version 219, Hibernate, a popular open source package for object/relational mapping, contains an excellent example of what not to do with command line input (Thanks to Yekaterina Tsipenyuk O Neil for pointing out this issue) The Java version of Hibernate s SchemaExport tool accepts a command-line parameter named "--delimiter", which it uses to separate SQL commands in the scripts it generated Example 53 shows how it works in a simplified form
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Example 53 Version 219 of Hibernate s SchemaExport tool allows SQL injection through the command line
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String delimiter; for (int i=0; i < argslength; i++) { if ( args[i]startsWith("--delimiter=") ) { delimiter = args[i]substring(12); } } for (int i = 0; i < dropSQLlength; i++) { try { String formatted = dropSQL[i]; if (delimiter!=null) formatted += delimiter; fileOutputwrite( formatted + "\n" ); }
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The --delimiter option exists so that a user can specify the separator that should appear between SQL statements Typical values might be a semicolon or a carriage return and a line feed But the program does not place any restrictions on the argument s value, so from a command-line parameter, you can write any string you want into the generated SQL script, including additional SQL commands For example, if a simple SELECT query was provided with --delimiter ';', it would generate a script to execute the following command:
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SELECT * FROM items WHERE owner = "admin";
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But if the same query was issued with the malicious option --delimiter
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'; DELETE FROM items;', it would generate a script that cleans out the
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SELECT * FROM items WHERE owner = "admin"; DELETE FROM items;
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From a na ve perspective, this is of no consequence After all, if you wanted to execute a malicious query, you could always specify it directly, right This line of reasoning contains an implicit and dangerous set of assumptions about how the program will be used It is now incumbent upon any programmer who wants to write a wrapper script around SchemaExport to understand that the --delimiter command-line parameter affects a query in an unconstrained fashion The name delimiter suggests that the value should be something short, such as a piece of punctuation, but the program does no input validation at all; therefore, it is not acceptable to give control of this parameter to someone who is not authorized to write arbitrary commands into the output le Want to write a Web front end for provisioning a new database This code makes it easy for that new front end to unwittingly turn complete control of the new database over to the provisioner because now anyone who controls the input to SchemaExport can insert arbitrary SQL commands into the output This is a security meltdown waiting to happen Database Queries Unlike input received directly from an anonymous user, information from the database must often be granted a level of trust In many cases, it is impossible to verify that data from the database are correct because the database is often the only source of truth On the other hand, programs that rely on the database should verify that information retrieved from the database is well formed and meets reasonable expectations Do not blindly rely on the database to ensure that your application will behave correctly The following are just two examples of validation that can be performed on database data: Check to make sure that only one row exists for values that are expected to be unique The presence of two entries might indicate that an attacker managed to insert a falsi ed data entry Database features, such as triggers or uniqueness constraints, might not be in effect For example, you might nd that a user has two entries indicating their account balance The code in Example 54 makes no effort to verify the number of rows returned by the database; it simply uses the rst row found Example 55 gives a revised version that checks to make sure the database returns only one row
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