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Executable-space protection
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<p>In <a href="/facts/Computer_security/7E5dIy7G">computer security</a>, executable-space protection marks <a href="/facts/Computer_memory/hQKVQ51J">memory</a> regions as non-executable, such that an attempt to execute <a href="/facts/Machine_code/bgdQ1Fxf">machine code</a> in these regions will cause an <a href="/facts/Exception_handling/VrJxgb9r">exception</a>. It makes use of hardware features such as the <a href="/facts/NX_bit/MnBB93BK">NX bit</a> (no-execute bit), or in some cases software emulation of those features. However, technologies that emulate or supply an NX bit will usually impose a measurable overhead while using a hardware-supplied NX bit imposes no measurable overhead. </p><p>The <a href="/facts/Burroughs_large_systems/FwY7EqrB">Burroughs 5000</a> offered hardware support for executable-space protection on its introduction in 1961; that capability remained in its successors until at least 2006. In its implementation of <a href="/facts/Tagged_architecture/RQfWwKTH">tagged architecture</a>, each word of memory had an associated, hidden tag bit designating it code or data. Thus user programs cannot write or even read a program word, and data words cannot be executed. </p><p>If an <a href="/facts/Operating_system/8XTuvMh2">operating system</a> can mark some or all writable regions of memory as non-executable, it may be able to prevent the <a href="/facts/Call_stack/1sMt713v">stack</a> and <a href="/facts/Dynamic_memory_allocation/vthLkUpz">heap</a> memory areas from being executable. This helps to prevent certain <a href="/facts/Buffer_overflow/uMFduL4B">buffer overflow</a> <a href="/facts/Exploit_(computer_security)/EdlqsRJM">exploits</a> from succeeding, particularly those that inject and execute code, such as the <a href="/facts/Sasser_(computer_worm)/K4SatnXK">Sasser</a> and <a href="/facts/Blaster_(computer_worm)/HbkJadL5">Blaster</a> worms. These attacks rely on some part of memory, usually the stack, being both writable and executable; if it is not, the attack fails. </p>