Because of the nature of barcodes, developers may not be expecting attacks from that vector and thus don't sanitize their inputs properly. I had previously written "XSS, Command and SQL Injection vectors: Beyond the Form" so this was right up my alley. I constructed this page that lets you make barcodes in Code 93, Code 39, Code 39ext and Code 128A, B and C. Link: http://www.irongeek.com/xss-sql-injection-fuzzing-barcode-generator.php