Yes.
The only thing identifying a user is a pseudo-random value being sent along with each request.If an attacker can guess the right values to send, he can pose as somebody else.
There are different ways to make this harder:
- make session ids longer (more entropy, harder to guess)
- check additional information like the user agent (essentially more entropy)
- obviously: use a good random number generator
- expire sessions sooner to give a smaller set of valid session ids at any one time
- renew session ids often, even for valid ids
- use SSL to encrypt all communication to avoid outright cookie hijacking
