This is a negotiable point and reference should be made to applicable leasing legislation. Most leases will contain a clause which provides that if the landlord notices a breach or default in the lease, it may (upon notice to the tenant, or as required in the lease) avail itself of the funds from the security deposit to remedy such breach or default. The tenant will usually be obligated to top up the security deposit (to the original amount) within a certain period of time.