How does venom work exactly

- Is it passive?
- Does it add attack like warrior potion?
- Does the duration of the poison last 4 seconds or if the skill is active does it only last 4 seconds?
- What Duel Blade skills is it limited to?

It's passive, does not add attack (the attack listed in the skill is part of the Venom damage formula), and the duration of the poison is 4 seconds.

Venom damage formula