If any of these variables is used to execute dangerous commands (such as direct commands to a SQL database or the host computer operating system), the taint checker warns that the program is using a potentially dangerous tainted variable.
The computer programmer can then redesign the program to erect a safe wall around the dangerous input.
Taint checking may be viewed as a conservative approximation of the full verification of non-interference or the more general concept of secure information flow.
[6] The following dangerous Perl code opens a large SQL injection vulnerability by not checking the value of the $name variable: If taint checking is turned on, Perl would refuse to run the command and exit with an error message, because a tainted variable is being used in a SQL query.
One thing to note is that Perl DBI requires one to set the TaintIn attribute of a database handle as well as enabling taint mode to check one's SQL strings.