Is a common carrier liable for the acts of strangers or criminals?

Yes. A common carrier is liable even for acts of strangers like thieves or robbers.

Exception: where such thieves or robbers acted "with grave or irresistible threat, violence or force." The common carrier is not liable for the value of the undelivered merchandise which was lost because of an event that is beyond his control. (De Guzman v. CA, G.R. No. L-47822, Dec. 22, 1988)