một bước riêng biệt. Diffing có thể được thực hiện tại đĩa, tập tin, và mức độ cơ sở dữ liệu. Ở cấp độ đĩa, bạn có thể khám phá ra các tập tin đã được sửa đổi. Ở cấp độ tập tin, bạn có thể khám phá ra những byte đã được thay đổi. Ở cấp cơ sở dữ liệu, bạn có thể khám phá ra những | Unexpected Input Chapter 7 213 into a table that she can read from directly. And of course the query may not need to be seen such as a DROP command. Figure Some Database Servers such as Microsoft SQL Server Allow for Multiple SQL Commands in One Query When trying to submit extra commands the attacker may need to indicate to the data server that it should ignore the rest of the query. Imagine a query such as this SELECT FROM table WHERE x data AND z 4 Now if you submit the same data as mentioned earlier the query would become this . WHERE x 1 SELECT FROM table WHERE y 5 AND z 4 This results in the AND z 4 being appended to the second query which may not be desired. The solution is to use a comment indicator which is different with every database some may not have any . On MS SQL Server including a double hyphen tells the database to ignore the rest as shown in Figure . 214 Chapter 7 Unexpected Input On MySQL the pound sign is the comment character. So for a MySQL server an attacker would submit 1 SELECT FROM table WHERE y 5 which results in the following final query of . WHERE x 1 SELECT FROM table WHERE y 5 AND z 4 causing the server to ignore the AND z 4. Figure Escaping the First Query by Submitting blah select from sales - Which Makes Use of the Comment Indicator -- in MS SQL Server In these examples you know the name of your target table which is not always the may have to know table and column names in order to perform valid SQL queries because this information typically isn t publicly accessible it can prove to be a crux. However all is not databases have different ways to query system information to gain lists of installed tables. For example querying the sysobjects table with a Select from sysobjects query in Unexpected Input Chapter 7 215 Microsoft SQL Server will return all objects registered for that database including stored procedures and table names. When involved in SQL hacking it s .
