How likely is transmitting hiv through oral sex with a canker sore present?

Bad idea: If the other person has previous been exposed to the same virus then they should be immune, however sores are an active out break of a viral infection and contact with others to the sore in any form should be avoided.

There is a chance: Hiv is a blood-borne disease, so any open sores or breaks in the skin/gums can transmit fluids that carry the virus.