If a developer writes a cross-site scripting error as they're coding in JavaScript, and they're able to detect that within minutes of creating that flaw, it will likely only require minutes or seconds to fix.

Whereas if that flaw is discovered two weeks later by a manual tester, that's going to be then entered into a defect tracking system. It's going to be triaged. It's going to be put into someone's bug queue.

With the delay in identification, it will have to be researched in its original context and will slow down development. Now, you're potentially talking hours of time to fix the same flaw. Maybe a scale of 10 or 100 times more time is taken

I couldn't agree with Chris Wysopal (CTO, Veracode) more in his recent podcast with O'Reilly Security Podcast where he explains why shifting security to the left (to developers at the start of the development life cycle) is key in an agile environment to keep up the pace and speed.

Security should be made easy for developers by using IDE plug-ins, scanners and educating them to have the basic security skills (hygiene). Organisations should not be solely relying on security experts or a centralised security team who validates all changes.

Our typical security modus operandi is broken (call in the expert!) and we need to integrate security into development teams to ensure quality is maintained whilst staying agile.

https://www.oreilly.com/ideas/chris-wysopal-on-a-shared-responsibility-model-for-developers-and-defenders