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Address Space Layout Randomization (ASLR) changes where selected parts of a program or operating system are placed in memory. That makes attacks that depend on a known address harder to carry out reliably. ASLR does not fix the bug an attacker may exploit, and it cannot guarantee that exploitation will fail.
Why knowing a memory address can help an attacker
A running program uses virtual addresses to refer to its code and data. These addresses identify locations in the process’s address space; they are not necessarily fixed physical locations in RAM. An exploit that redirects execution or locates useful code may work more reliably if it can count on a function, library, stack, heap, or mapped region being at a predictable address.
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ASLR varies the locations of selected regions. If an attack uses an address that was valid in one run, that address may point somewhere else in another. The attacker must then find or infer the new location, making an address-dependent exploit less reliable. Ubuntu’s ASLR documentation describes the Linux kernel and ELF loader as sharing this work: the kernel randomizes aspects of the initial process layout, while the loader places executables and shared libraries at varying locations.
What ASLR changes—and what it does not
ASLR is about uncertainty in memory layout, not removal of the underlying vulnerability. It can complicate techniques such as return-to-libc attacks, which redirect a program to existing code at an address the attacker needs to know. But ASLR does not make unsafe code safe, and it does not prevent every attack. Its protection depends on which regions are randomized, how much variation the implementation provides, and whether an attacker can discover the addresses by other means.
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In this context, entropy means the uncertainty in a possible location: more possible placements can make guessing harder. It is not a single universal ASLR score. Address-space size, platform, configuration, and implementation all affect the amount of uncertainty available.
How ASLR differs across platforms
“ASLR” refers to a family of protections, not one identical setting on every system. Which regions move, and when they move, depends on the operating system and its configuration.
Linux user processes
Ubuntu documents several regions that may be randomized: the stack, shared-library and mmap locations, position-independent executables, the brk heap, and the vDSO. Its documented /proc/sys/kernel/randomize_va_space settings are:
| Value | Documented effect |
|---|---|
0 |
Disables ASLR. |
1 |
Randomizes the stack, mmap base, and vDSO. |
2 |
Adds heap randomization to the regions covered by value 1. |
These are Ubuntu’s documented settings, not a promise about every Linux distribution. Ubuntu says value 2 is the default on most systems when CONFIG_COMPAT_BRK is disabled; when that option is enabled, the default is value 1. Executables built as position-independent executables (PIE) with -fPIE -pie can be loaded at varying locations.
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Linux kernel: KASLR
Kernel ASLR, or KASLR, protects kernel space rather than an individual user process. Linux documentation describes randomizing the kernel’s physical and virtual base at boot, as well as offsets for module bases, kernel stack bases, and dynamic-memory bases. The documentation treats structure-layout randomization separately: it is applied per build, rather than being the same kind of per-process or per-boot placement change. The Linux kernel’s self-protection guidance also stresses limiting disclosures of kernel addresses and contents, because they can reveal locations that randomization is meant to obscure.
Windows
Microsoft distinguishes several Windows exploit-protection controls. Mandatory ASLR rebases images, but rebasing alone can still place an image predictably; Microsoft says it should be paired with Bottom-up ASLR, which adds variation to allocation locations. The available address space also limits how much entropy can be used, especially for 32-bit applications.
Microsoft’s Exploit protection reference documents a high-entropy Bottom-up ASLR option providing 24 bits of entropy—described as 1 TB of variance—for 64-bit applications. This figure applies to that specific option, not to all Windows ASLR settings or to other systems. Microsoft also notes a compatibility risk: some applications truncate pointers into 32-bit variables because they expect addresses below 4 GB.
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Apple’s platform-security guide, published December 19, 2024, says iOS, iPadOS, and visionOS use ASLR as part of runtime security. It describes randomization of executable code, system libraries, and related constructs, and says Xcode and the iOS/iPadOS development environments automatically compile third-party programs with ASLR support enabled. Apple presents ASLR alongside sandboxing, entitlements, and Execute Never protections.
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Why ASLR can be defeated or weakened
Address disclosures
Randomization makes addresses harder to predict, but an information leak can disclose a location directly or provide enough information to infer it. Linux kernel guidance notes that address and content disclosures can make otherwise hidden locations more useful to an attacker. ASLR therefore works best when software also avoids leaking sensitive memory information.
Finite address space and implementation choices
Randomization has limits: only selected regions move, and the available address space bounds the possible locations. A 32-bit process has less room for variation than a 64-bit process. A 2024 study by Binosi, Barzasi, Carminati, Zanero, and Polino evaluated Linux, macOS, and Windows and reported differences among the tested platforms, including limitations for some tested areas and reduced library entropy after Linux 5.18. Those are findings for the study’s methods and tested versions, not a blanket description of every current installation. See the ACM CCS 2024 paper for its platform- and version-specific results.
Where ASLR fits in a security strategy
ASLR is one layer in defense in depth, not a complete security boundary. Microsoft describes security features as sometimes providing protection without a robust defense against every threat. Apple’s platform guidance likewise presents ASLR with controls such as sandboxing and Execute Never. Other mitigations address different parts of the problem: memory-safe coding practices reduce the chance of certain memory errors, while execution protections can limit what an attacker can do with corrupted memory.
The useful question is not simply whether a system “has ASLR,” but which regions are randomized, when that happens, how much uncertainty is available, whether addresses can leak, and what companion protections are enabled. Defaults and settings vary by platform and configuration, so the presence of an ASLR feature alone does not establish how strongly a particular application is protected.
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