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How ASLR protects Linux systems from buffer overflow attacks

ASLR (Address Space Layout Randomization) is a memory exploitation mitigation technique used on both Linux and Windows systems. Learn how to tell if it’s running, enable/disable it, and get a view of how it works.

Address Space Layout Randomization (ASLR) is a memory-protection process for operating systems that guards against buffer-overflow attacks. It helps to ensure that the memory addresses associated with running processes on systems are not predictable, thus flaws or vulnerabilities associated with these processes will be more difficult to exploit.

ASLR is used today on Linux, Windows, and MacOS systems. It was first implemented on Linux in 2005. In 2007, the technique was deployed on Microsoft Windows and MacOS. While ASLR provides the same function on each of these operating systems, it is implemented differently on each one.

The effectiveness of ASLR is dependent on the entirety of the address space layout remaining unknown to the attacker. In addition, only executables that are compiled as Position Independent Executable (PIE) programs will be able to claim the maximum protection from ASLR technique because all sections of the code will be loaded at random locations. PIE machine code will execute properly regardless of its absolute address.

ASLR limitations

In spite of ASLR making exploitation of system vulnerabilities more difficult, its role in protecting systems is limited. It’s important to understand that ASLR:

  • Doesn’t resolve vulnerabilities, but makes exploiting them more of a challenge
  • Doesn’t track or report vulnerabilities
  • Doesn’t offer any protection for binaries that are not built with ASLR support
  • Isn’t immune to circumvention

How ASLR works

ASLR increases the control-flow integrity of a system by making it more difficult for an attacker to execute a successful buffer-overflow attack by randomizing the offsets it uses in memory layouts.

ASLR works considerably better on 64-bit systems, as these systems provide much greater entropy (randomization potential).

Is ASLR working on your Linux system?

Either of the two commands shown below will tell you whether ASLR is enabled on your system.

$ cat /proc/sys/kernel/randomize_va_space
$ sysctl -a --pattern randomize
kernel.randomize_va_space = 2

The value (2) shown in the commands above indicates that ASLR is working in full randomization mode. The value shown will be one of the following:

0 = Disabled
1 = Conservative Randomization
2 = Full Randomization

If you disable ASLR and run the commands below, you should notice that the addresses shown in the ldd output below are all the same in the successive ldd commands. The ldd command works by loading the shared objects and showing where they end up in memory.

udo sysctl -w kernel.randomize_va_space=0	<== disable
[sudo] password for shs:
kernel.randomize_va_space = 0
$ ldd /bin/bash (0x00007ffff7fd1000) <== same addresses => /lib/x86_64-linux-gnu/ (0x00007ffff7c69000) => /lib/x86_64-linux-gnu/ (0x00007ffff7c63000) => /lib/x86_64-linux-gnu/ (0x00007ffff7a79000)
        /lib64/ (0x00007ffff7fd3000)
$ ldd /bin/bash (0x00007ffff7fd1000) <== same addresses => /lib/x86_64-linux-gnu/ (0x00007ffff7c69000) => /lib/x86_64-linux-gnu/ (0x00007ffff7c63000) => /lib/x86_64-linux-gnu/ (0x00007ffff7a79000)
        /lib64/ (0x00007ffff7fd3000)
If the value is set back to 2 to enable ASLR, you will see that the addresses
 will change each time you run the command.
$ sudo sysctl -w kernel.randomize_va_space=2	<== enable
[sudo] password for shs:
kernel.randomize_va_space = 2
$ ldd /bin/bash (0x00007fff47d0e000) <== first set of addresses => /lib/x86_64-linux-gnu/ (0x00007f1cb7ce0000) => /lib/x86_64-linux-gnu/ (0x00007f1cb7cda000) => /lib/x86_64-linux-gnu/ (0x00007f1cb7af0000)
        /lib64/ (0x00007f1cb8045000)
$ ldd /bin/bash (0x00007ffe1cbd7000) <== second set of addresses => /lib/x86_64-linux-gnu/ (0x00007fed59742000) => /lib/x86_64-linux-gnu/ (0x00007fed5973c000) => /lib/x86_64-linux-gnu/ (0x00007fed59552000)
        /lib64/ (0x00007fed59aa7000)

Attempting to bypass ASLR

In spite of its advantages, attempts to bypass ASLR are not uncommon and seem to fall into several categories:

  • Using address leaks
  • Gaining access to data relative to particular addresses
  • Exploiting implementation weaknesses that allow attackers to guess addresses when entropy is low or when the ASLR implementation is faulty
  • Using side channels of hardware operation


ASLR is of great value, especially when run on 64 bit systems and implemented properly. While not immune from circumvention attempts, it does make exploitation of system vulnerabilities considerably more difficult. Here is a reference that can provide a lot more detail on the Effectiveness of Full-ASLR on 64-bit Linux, and here is a paper on one circumvention effort to bypass ASLR using branch predictors.


Side-Channel Attack Targets Windows, Linux

A research team of experts from Graz University of Technology, Boston University, NetApp, CrowdStrike, and Intel has published findings on page cache attacks. Unlike Spectre and Meltdown, this attack is a first-of-its-type, hardware-agnostic, side-channel attack that can remotely target operating systems such as Windows and Linux and effectively exfiltrate data, bypassing security precautions.

In explaining the attack, authors wrote: “Our side-channel permits unprivileged monitoring of some memory accesses of other processes, with a spatial resolution of 4KB and a temporal resolution of 2 microseconds on Linux (restricted to 6.7 measurements per second) and 466 nanoseconds on Windows (restricted to 223 measurements per second); this is roughly the same order of magnitude as the current state-of-the-art cache attacks.”

After detailing background information on hardware caches, cache attacks, and software caches, the authors provide an attack threat model in which the researchers “assume that attacker and victim have access to the same operating system page cache. On Linux, we also assume that the attacker has read access to the target page, which may be any page of any attacker-accessible file on the system.”

In addition to mitigation strategies, the researchers also stated that they responsibly disclosed the vulnerability to Microsoft, and the company said it will roll out a fix.

“This attack class presents a significantly lower complexity barrier than previous hardware-based, side-channel attacks and can easily be put into practice by threat actors, both nation-state as well as cyber-gangs,” said Mounir Hahad, head of Juniper Threat Labs at Juniper Networks.

“In particular, password recovery via unprivileged applications is a major worry, as it would be available to most unwanted software bundlers and other programs typically thought of as relatively harmless. There is not much that an end user can currently do to protect themselves against this type of attack except to not run any software from a shady source, even if it does not raise any antivirus flag,” said Hahad.