Nothing ventured, nothing gained, but it's better to remain silent and thought a fool than to speak up and remove all doubt.
Shy people can make it through life content, but not happy. Shy people make up middle management. Outgoing people can either stay at the bottom or rise to the top depending on their other personality traits.
To answer the question directly, security can be gained by being shy, but ambition cannot. I'd rate it as a more negative personality trait than others, but certainly not the worst. It's something you can change about yourself if you're willing to. The further away from grade school you get, the less shy most people become.