Facebook is doubling the rewards it will pay for security vulnerabilities related to code that runs its advertising system, the company said Wednesday. A comprehensive security audit of its ads code was recently completed, but Facebook “would like to encourage additional scrutiny from whitehats to see what we may have missed,” wrote Collin Greene, a security engineer, in a blog post. “Whitehats” refers to ethical security researchers, as opposed to “blackhats” who take advantage of vulnerabilities.