Al Shabaab is mostly funded by its own ability to tax everything and everyone in Somalia. It has a very effective network called the Amniyat that knows every Somali civil servant, every business person, and every plantation owner. They tax them systematically. Not paying the tax is not an option, because that will create a significant security problem for you. It's an extortion racket, but it's a very, very effective one. What that means is that any resources introduced into Somalia, whether by the Somali diaspora, USAID, or the World Bank, al Shabaab is getting a cut, and they're quite good at figuring out how to do that.
They do not have major flows of external money coming from, for instance, al Qaeda. The diaspora, once, years back, was a more significant source of funding, but that's largely dried up. This is a pretty self-sufficient group. We do hear that they have received funding from some interest in the gulf, but we don't know how significant that is. I don't think it's decisive. Given how much they're able to tax in Somalia, they have more than enough resources to conduct the kind of asymmetrical urban guerrilla war and terrorist attacks they're currently doing.