If a tenant breaks their lease - are they entitled to their security deposit back?

Depends on what the lease states. And HOW the lease was broken. I broke my lease on the best of terms possible, and yes I got my deposit back.

We would really need to know all of the details.Breaking a lease can mean many things to many people. If you had a 1 year lease and walked out 6 months early you still owe 6 months rent or until a new tenant moves in. Maybe a LL would negotiate something more lienient than this but most will not. Yes they can apply the security to any unpaid rent.

Generally a security deposit can be used for two purposes;
a) to cover damages to the premises
b) to cover unpaid rental

If a tenant break a lease, then there is a good chance they will have unpaid rental. Unless they had an agreement with the landlord to release the tenant from the lease, then they will owe rental until the lease expires or the landlord rents the unit to a new tenant.

If you are the tenant you need to try and reach an agreement with the landlord. If you are the landlord, then you still need to provide the tenant with documentation of how the deposit was used within the statutory period (generally 30 days, but might be less).

If you want more help, please post more details, including where you are located so we can help further.